fix(i18n): numbers, units, dates and order in the UI language, and after a switch (E30 S3)

Every locale formatter takes the language as an argument. Under the React
Compiler a formatter that read the i18n singleton was memoized on its other
arguments, so a card kept "2 hr 4 min" after a switch to Hungarian; plexLang()
had the same flaw and left Plex queries in the old language. Components now
take the language from useTranslation, and the signature makes forgetting it
a type error.

- ratings, file sizes, stream stats: the language's decimal mark ("8,3")
- view and subscriber counts compact in the language ("1,2 E")
- seconds as the language's unit ("6 mp") in the sliders and the stats
- a person's birth and death dates as dates, not ISO strings
- A-Z lists by the shown name in the language's alphabet (compareText),
  the tag catalog re-sorted by its translated names
- lint local/locale-format (was date-format) also refuses Intl.NumberFormat
  outside lib/format.ts, localeCompare, and a component reading the language
  off the i18n singleton

docs/DESIGN.md

@@ -531,7 +531,7 @@ pick a density step; it never changes the anatomy or loses an action. [Nielsen #
 - **Messages.** A toast or error of one sentence has no full stop and no "✓": "Couldn't sync
   subscriptions" / "Nem sikerült szinkronizálni". Two sentences keep their stops. Help text is prose.
 - **Quotes** around a name are curly: “abc” in English, „abc” in Hungarian.
-- **Dates**, in the UI language, only from `lib/format.ts` (lint: `local/date-format`): recency in a list
+- **Dates**, in the UI language, only from `lib/format.ts` (lint: `local/locale-format`): recency in a list
   is relative ("3d ago" / "3 napja") through `ui/RelativeTime`, which carries the exact minute on hover;
   a date is "Sep 27, 2026" / "2026. szept. 27."; a moment "Sep 27, 2026, 08:30 AM" /
   "2026. szept. 27. 08:30"; seconds only in the server log, the audit and the recorder.
@@ -540,6 +540,12 @@ pick a density step; it never changes the anatomy or loses an action. [Nielsen #
   label holding one is written in that same form ("Under 90 min"). Never a bare letter ("90m", "24h",
   "7d"; test-held). In a sentence, and in a list of time spans named in words ("24 hours", "1 week"), the
   unit is a word. A clock-style length on a picture stays "2:34:11".
+- **Numbers** in the UI language, from `lib/format.ts` (same lint): the decimal mark and grouping are the
+  language's ("8.3" / "8,3", "1,234" / "1 234"); a big count is compact ("1.2K" / "1,2 E"); a size keeps
+  its symbol ("5.0 GB" / "5,0 GB"), as do a stream's (Mbps) and "%" / "px". Never `toFixed` on screen.
+- **Order.** An A→Z list is in the UI language's alphabet ("Ö" after "O", "Cs" after "Cu" in Hungarian),
+  by the name the reader SEES: `compareText` (`lib/collate.ts`) on the client, the ICU collation of
+  `app/collation.py` on the server, so both sort alike.
 - A label says what happens ("Delete playlist"), not "OK"; a destructive confirm names the object.
 
 ## Anti-patterns
@@ -624,6 +630,7 @@ pick a density step; it never changes the anatomy or loses an action. [Nielsen #
 | 2026-09-27 | C4 as built                                      | `initialFocus` required on every `Modal`; the commit row in `ModalFooter`, portalled under the body from any depth; the fades driven by the body's scroll timeline; `interactive-widget=resizes-content`                                                                                                               | An optional prop defaulting to the ✕; a `footer` prop (the issue composer's buttons live three components down); always-on fades                                                                       | A default is how 21 dialogs came to open on the ✕; lifting the composer's state to reach a prop is the larger change; a resting fade dimmed the first line                                    |
 | 2026-09-27 | C3 + C5 as built                                 | A tap toggles the bar through `pointerType` from `pointerdown`; a double-tap takes back its first tap's toggle; the card's pair is the video card's `MediaResumeButtons` (Continue / Restart), a sibling over the picture; the YT description is an ⓘ click popover                                                    | A delayed single tap (wait out the double-tap window); new "Resume / From the start" words for Plex; the pair inside the picture's button                                                              | A delayed tap feels dead; one media card says one pair of words; a button inside a button is invalid and the hover row already sits beside the picture                                        |
 | 2026-09-27 | C2 as built                                      | The side panel's drawer is `ui/Modal` `drawer-start`; drawer = a coarse pointer or below 768; the pin offered in its header ≥ 1024                                                                                                                                                                                     | A drawer of its own inside `SidePanel`                                                                                                                                                                 | Esc, Back, the scrim, the trap and the scroll lock are the dialog's; a second implementation is how two shells drift                                                                          |
+| 2026-09-29 | Numbers and order in the UI language (E30 S3)    | Every number through `lib/format.ts` in the UI language (decimal mark, grouping, compact, units); A→Z by the shown name in the UI language's alphabet, client and server alike; the language comes off `useTranslation`, never the singleton                                                                           | `toFixed`, hand-built "1.2K", `localeCompare` in the browser's locale, byte-order server sorts                                                                                                         | A Hungarian reader saw "8.3", "1.2K", "Életrajzi" after "Z"; the singleton read also left values in the old language after a switch (React Compiler)                                          |
 | 2026-09-27 | Narrow rows by container                         | The download row, the playlist table and the Channels tag row stack by a container query; a fact line never wraps                                                                                                                                                                                                      | Breakpoints on the window                                                                                                                                                                              | The iPad zoomed in, split screen and a column beside a rail are narrow while the window is not; the zoomed download row broke "18.9 / MB" apart                                               |
 | 2026-09-27 | Phones deferred                                  | A phone-only layout (the hamburger nav, a folded feed band) waits for its own epic; E28 stays desktop + tablet                                                                                                                                                                                                         | Fix the phone views inside E28                                                                                                                                                                         | Principle 2; the first phone attempts made the feed worse, and the epic's review grows with every sprint                                                                                      |
 | 2026-09-27 | Conformance check (S3g)                          | A four-step check before a UI change is done: primitive at a named rung, the chapter re-read, the rendered page measured by kind and look, every state seen                                                                                                                                                            | A reviewer's eye per sprint                                                                                                                                                                            | The S1 probe found 69 button looks; after S3 every control kind is drawn by its primitive, and the check keeps a new look from arriving unnoticed                                             |

frontend/eslint.config.js

@@ -82,33 +82,66 @@ const focusSafeDisabled = {
     };
   },
 };
-// E28 S3g (docs/DESIGN.md "Copy"): the app shows a date in three forms plus the relative one, all
-// built in lib/format.ts in the UI language. A bare `toLocaleString()` formats in the BROWSER's locale
-// and as numbers with seconds; seven call sites had drifted into five shapes. Outside lib/format.ts
-// a `toLocale*String` call or an `Intl.DateTimeFormat` / `Intl.RelativeTimeFormat` is refused (a
-// number goes through formatNumber, which the same rule covers).
-const dateFormat = {
+// E28 S3g + E30 S3 (docs/DESIGN.md "Copy"): a date, a number and an alphabetical order are all in the
+// UI language — dates and numbers built in lib/format.ts, text compared in lib/collate.ts. A bare
+// `toLocaleString()` formats in the BROWSER's locale (seven date sites had drifted into five shapes), a
+// bare `toFixed` writes the English decimal point, a bare `localeCompare` sorts in the browser's
+// alphabet. So outside those files a `toLocale*String` call or an Intl date/number formatter is refused,
+// an `Intl.Collator` outside collate.ts, and a `localeCompare` anywhere.
+//
+// And in a component, the language itself must come off `useTranslation()`, never the i18n singleton
+// (`import i18n from "../i18n"`): the React Compiler memoizes on what it can see, and a singleton read
+// is invisible to it — a card kept "2 hr 4 min" and a Plex query its English after a switch to
+// Hungarian. The singleton is resolved by SCOPE, so `const { i18n } = useTranslation()` passes.
+const localeFormat = {
   meta: {
     type: "problem",
     messages: {
-      date: "Format dates and numbers through lib/format.ts (formatDate, formatDateTime, formatDateTimeSec, relativeTime, formatNumber), in the UI language.",
+      date: "Format dates and numbers through lib/format.ts (formatDate, formatDateTime, formatNumber, formatDecimal, …), in the UI language.",
+      order:
+        "Compare text with compareText (lib/collate.ts), in the UI language — a bare localeCompare sorts in the browser's.",
+      singleton:
+        "Take the language from useTranslation() (`const { i18n } = useTranslation()`), not the i18n singleton — the React Compiler cannot see a singleton read, so the value stays in the old language after a switch.",
     },
   },
   create(context) {
     const path = context.filename.split("\\").join("/");
-    // lib/format.ts is where the forms live; a test computes its expectations; the Plex player's
-    // on-screen clock (lib/plexPlayerPrefs.ts) is a clock face with its own 12/24h switch, not a date.
-    if (/\/src\/lib\/(format|plexPlayerPrefs)\.ts$|\.test\.tsx?$/.test(path)) return {};
+    if (/\.test\.tsx?$/.test(path)) return {};
+    // lib/format.ts is where the forms live; the Plex player's on-screen clock (lib/plexPlayerPrefs.ts)
+    // is a clock face with its own 12/24h switch, not a date.
+    const formats = /\/src\/lib\/(format|plexPlayerPrefs)\.ts$/.test(path);
+    const collates = /\/src\/lib\/collate\.ts$/.test(path);
+    const component = /\/src\/components\//.test(path);
+    const importedFromI18n = (node) => {
+      for (let scope = context.sourceCode.getScope(node); scope; scope = scope.upper) {
+        const v = scope.set.get(node.name);
+        if (v)
+          return v.defs.some(
+            (d) => d.type === "ImportBinding" && /\/i18n$|^i18next$/.test(d.parent.source.value),
+          );
+      }
+      return false;
+    };
     return {
       "CallExpression > MemberExpression.callee[property.name=/^toLocale(Date|Time)?String$/]"(
         node,
       ) {
-        context.report({ node, messageId: "date" });
+        if (!formats) context.report({ node, messageId: "date" });
       },
-      "MemberExpression[object.name='Intl'][property.name=/^(DateTimeFormat|RelativeTimeFormat)$/]"(
+      "MemberExpression[object.name='Intl'][property.name=/^(DateTimeFormat|RelativeTimeFormat|NumberFormat)$/]"(
         node,
       ) {
-        context.report({ node, messageId: "date" });
+        if (!formats) context.report({ node, messageId: "date" });
+      },
+      "MemberExpression[object.name='Intl'][property.name='Collator']"(node) {
+        if (!collates) context.report({ node, messageId: "order" });
+      },
+      "CallExpression > MemberExpression.callee[property.name='localeCompare']"(node) {
+        context.report({ node, messageId: "order" });
+      },
+      "MemberExpression[property.name='language'][object.type='Identifier']"(node) {
+        if (component && importedFromI18n(node.object))
+          context.report({ node, messageId: "singleton" });
       },
     };
   },
@@ -382,7 +415,7 @@ const localRules = {
     "jsx-comment-text": jsxCommentText,
     "focus-safe-disabled": focusSafeDisabled,
     "button-primitive": buttonPrimitive,
-    "date-format": dateFormat,
+    "locale-format": localeFormat,
     "input-primitive": inputPrimitive,
     "surface-role": surfaceRole,
     "literal-a11y-text": literalA11yText,
@@ -477,7 +510,7 @@ export default tseslint.config(
       "local/jsx-comment-text": "error",
       "local/focus-safe-disabled": "error",
       "local/button-primitive": "error",
-      "local/date-format": "error",
+      "local/locale-format": "error",
       "local/input-primitive": "error",
       "local/surface-role": "error",
       "local/literal-a11y-text": "error",

frontend/src/components/About.tsx

@@ -8,10 +8,10 @@ import { formatDateTime } from "../lib/format";
 import { Button } from "./ui/Button";
 import Modal from "./ui/Modal";
 
-function fmtDate(d?: string | null): string {
+function fmtDate(d: string | null | undefined, lang: string): string {
   if (!d) return "—";
   const t = new Date(d);
-  return isNaN(t.getTime()) ? d : formatDateTime(d);
+  return isNaN(t.getTime()) ? d : formatDateTime(d, lang);
 }
 
 // About dialog: app + build + schema versions (frontend/backend/database), plus a
@@ -23,7 +23,8 @@ export default function About({
   onClose: () => void;
   onOpenReleaseNotes: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const { data } = useQuery({
     queryKey: qk.version(),
     queryFn: api.version,
@@ -40,7 +41,7 @@ export default function About({
       data ? data.app_version + (data.git_sha !== "unknown" ? ` · ${data.git_sha}` : "") : "…",
     ],
     [t("about.database"), data?.db_revision ?? "…"],
-    [t("about.buildDate"), fmtDate(data?.build_date ?? FRONTEND_BUILD_DATE)],
+    [t("about.buildDate"), fmtDate(data?.build_date ?? FRONTEND_BUILD_DATE, lang)],
   ];
 
   return (

frontend/src/components/AccountPanel.tsx

@@ -55,7 +55,8 @@ export default function AccountPanel({
   user: AdminUserRow;
   onClose: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [link, setLink] = useState<{ url: string; expires_at: string | null } | null>(null);
   const [copied, setCopied] = useState(false);
@@ -182,7 +183,7 @@ export default function AccountPanel({
               label={t("users.account.lastLogin")}
               value={
                 a.last_login_at
-                  ? formatDateTime(a.last_login_at)
+                  ? formatDateTime(a.last_login_at, lang)
                   : a.is_demo
                     ? t("users.account.lastLoginDemo")
                     : t("users.account.lastLoginNone")

frontend/src/components/AdminLogs.tsx

@@ -74,7 +74,8 @@ function toQuery(state: LogsUrlState, q: string): LogQuery {
 }
 
 export default function AdminLogs() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [state, setState] = useState<LogsUrlState>(() =>
     logsParamsToState(new URLSearchParams(window.location.search)),
@@ -406,7 +407,7 @@ export default function AdminLogs() {
                     level={row.level}
                     event={row.event ?? "—"}
                     msg={row.msg}
-                    title={formatDateTimeSec(row.ts)}
+                    title={formatDateTimeSec(row.ts, lang)}
                     open={open === row.id}
                     onToggle={() => setOpen(open === row.id ? null : row.id)}
                   >
@@ -468,14 +469,15 @@ function RowDetail({
   onRequest: (id: string) => void;
   onJob: (id: number) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const pivotCls =
     "font-mono underline decoration-dotted underline-offset-2 hover:decoration-solid";
   return (
     <div className="flex flex-col gap-2">
       <dl className="grid grid-cols-[max-content_minmax(0,1fr)] gap-x-3 gap-y-0.5 font-sans text-xs">
         <dt>{t("adminLogs.fields.when")}</dt>
-        <dd className="tabular-nums">{formatDateTimeSec(row.ts)}</dd>
+        <dd className="tabular-nums">{formatDateTimeSec(row.ts, lang)}</dd>
         <dt>{t("adminLogs.fields.source")}</dt>
         <dd className="font-mono">
           {row.source} · {row.logger}

frontend/src/components/AdminUsers.dom.test.tsx

@@ -93,7 +93,7 @@ describe("the last sign-in column (E22 S1)", () => {
       </QueryClientProvider>,
     );
 
-    expect(await screen.findByText(relativeTime(at))).toBeTruthy();
+    expect(await screen.findByText(relativeTime(at, "en"))).toBeTruthy();
     expect(screen.getAllByText("—")).toHaveLength(2);
   });
 });

frontend/src/components/AdminUsers.tsx

@@ -103,7 +103,8 @@ export default function AdminUsers() {
 }
 
 function UsersRoles({ me }: { me: Me }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   const q = useQuery({ queryKey: qk.adminUsers(), queryFn: api.adminUsers });
@@ -336,14 +337,14 @@ function UsersRoles({ me }: { me: Me }) {
                 <Tooltip
                   hint={
                     u.last_login_at
-                      ? t("users.roles.lastLogin", { time: formatDateTime(u.last_login_at) })
+                      ? t("users.roles.lastLogin", { time: formatDateTime(u.last_login_at, lang) })
                       : u.is_demo
                         ? t("users.roles.lastLoginDemo")
                         : t("users.roles.lastLoginNone")
                   }
                 >
                   <span className="relative hidden sm:inline-block w-20 shrink-0 text-right text-2xs text-muted tabular-nums">
-                    {u.last_login_at ? relativeTime(u.last_login_at) : "—"}
+                    {u.last_login_at ? relativeTime(u.last_login_at, lang) : "—"}
                   </span>
                 </Tooltip>
               </div>
@@ -440,7 +441,8 @@ function PendingAccessRowView({
   onActivate: () => void;
   onResend: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   return (
     <div className={`${SURFACE.card} flex items-center gap-2 p-2.5`}>
       <div className="min-w-0 flex-1">
@@ -449,7 +451,7 @@ function PendingAccessRowView({
           {row.created_at && (
             <span>
               {t("users.pendingAccess.signedUp", {
-                time: formatDateTime(row.created_at),
+                time: formatDateTime(row.created_at, lang),
               })}
             </span>
           )}
@@ -536,6 +538,7 @@ function InviteLangPicker({
 
 function AdminInvites() {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   const me = useMe();
@@ -640,7 +643,7 @@ function AdminInvites() {
                 : "settings.invites.inviteConfirm",
               {
                 email: inv.email,
-                time: formatDateTime(inv.invited_at ?? new Date(0).toISOString()),
+                time: formatDateTime(inv.invited_at ?? new Date(0).toISOString(), lang),
               },
             )}
           </p>
@@ -744,7 +747,7 @@ function AdminInvites() {
                     hint={
                       i.invited_at
                         ? t("settings.invites.invitedAt", {
-                            time: formatDateTime(i.invited_at),
+                            time: formatDateTime(i.invited_at, lang),
                           })
                         : t("settings.invites.inviteHint")
                     }
@@ -931,7 +934,8 @@ function InviteRow({
   onDelete: () => void;
   busy: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   return (
     <div className={`${SURFACE.card} flex items-center gap-2 p-2.5`}>
       <div className="min-w-0 flex-1">
@@ -939,7 +943,7 @@ function InviteRow({
         {inv.requested_at && (
           <div className="text-2xs text-muted">
             {t("settings.invites.requested", {
-              time: formatDateTime(inv.requested_at),
+              time: formatDateTime(inv.requested_at, lang),
             })}
           </div>
         )}

frontend/src/components/AuditLog.tsx

@@ -13,12 +13,14 @@ import DataTable from "./ui/DataTable";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { auditColumns } from "./auditColumns";
 import { notifyActionError } from "../lib/actionErrors";
+import { compareText } from "../lib/collate";
 
 // Admin-only audit trail: an append-only who/what/when log of admin actions + scheduler run
 // summaries (never per-row). Read-only apart from Clear. Client-side sort/filter/paginate via
 // DataTable; the API returns the most recent window (capped) — the log stays small by design.
 export default function AuditLog() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   const q = useQuery({ queryKey: qk.adminAudit(), queryFn: () => api.adminAudit() });
@@ -26,9 +28,11 @@ export default function AuditLog() {
   const rows = useMemo(() => data?.items ?? [], [data]);
 
   const actionOptions = useMemo(() => {
-    const present = Array.from(new Set(rows.map((r) => r.action))).sort();
-    return present.map((a) => ({ value: a, label: t(`auditActions.${a}`, a) }));
-  }, [rows, t]);
+    // By what the reader sees — the named action, in their alphabet — not by the code.
+    return Array.from(new Set(rows.map((r) => r.action)))
+      .map((a) => ({ value: a, label: t(`auditActions.${a}`, a) }))
+      .sort((a, b) => compareText(a.label, b.label, lang));
+  }, [rows, t, lang]);
   const columns = useMemo(() => auditColumns(t, actionOptions), [t, actionOptions]);
 
   const clear = useMutation({

frontend/src/components/BootGate.tsx

@@ -104,6 +104,7 @@ export default function BootGate({ children }: { children: ReactNode }) {
     const account = meRef.current;
     if (account && getActiveAccount() !== account.id) setActiveAccount(account.id);
     const lang = account?.preferences?.language;
+    // eslint-disable-next-line local/locale-format -- a one-shot read in an effect, compared before adopting the account's language; nothing renders it, so there is no memoized value to go stale.
     if (typeof lang === "string" && isSupported(lang) && lang !== i18n.language) setLanguage(lang);
     // eslint-disable-next-line react-hooks/set-state-in-effect -- the adoption latch. It cannot be derived: the whole point is that the tree mounts only AFTER the pin and the language have been applied, and both are writes to things outside React (sessionStorage, i18n). A layout effect so the frame that first shows the app already has them.
     setReadyFor(identity);

frontend/src/components/ChannelDiscovery.tsx

@@ -41,7 +41,8 @@ export default function ChannelDiscovery({
   onOpenWizard: () => void;
   onViewChannel: (id: string, name: string) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   // The table/cards density is shared with the Subscriptions tab (ChannelsProvider). The sort is
@@ -148,7 +149,7 @@ export default function ChannelDiscovery({
       hideInCard: true,
       render: (c) => <AboutCell text={c.description} channelId={c.id} />,
     },
-    subsColumn<DiscoveredChannel>(t),
+    subsColumn<DiscoveredChannel>(t, lang),
     {
       key: "videos",
       header: t("channels.discovery.cols.totalVideos"),
@@ -157,7 +158,7 @@ export default function ChannelDiscovery({
       sortable: true,
       sortValue: (c) => c.video_count ?? -1,
       render: (c) => (
-        <span className="text-muted tabular-nums">{formatCountOrDash(c.video_count)}</span>
+        <span className="text-muted tabular-nums">{formatCountOrDash(c.video_count, lang)}</span>
       ),
     },
     {
@@ -186,7 +187,7 @@ export default function ChannelDiscovery({
   // computed for cards — DataTable sorts and slices internally, so doing it in table mode too would
   // copy+sort the whole list every render for a result nothing reads.
   const isCards = layout === "cards";
-  const sortedRows = isCards ? sortRows(rows, columns, sort) : rows;
+  const sortedRows = isCards ? sortRows(rows, columns, sort, lang) : rows;
   const {
     paged: pagedRows,
     pagerProps,

frontend/src/components/ChannelPage.tsx

@@ -35,7 +35,7 @@ import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { notifyActionError } from "../lib/actionErrors";
 import { api, DEFAULT_TAG_MODES, type FeedFilters, type Me, type ChannelDetail } from "../lib/api";
 import type { FeedView } from "../lib/feedView";
-import { channelYouTubeUrl, formatDate, formatViews } from "../lib/format";
+import { channelYouTubeUrl, formatDate, formatCompact } from "../lib/format";
 import { ChannelTagsControl } from "./ChannelTags";
 import { useChannelTagActions } from "../lib/useChannelTagActions";
 import { Button } from "./ui/Button";
@@ -70,6 +70,7 @@ export default function ChannelPage({
   onShowInFeed: () => void;
 }) {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const { closeChannel } = useNavigationActions();
   const qc = useQueryClient();
   const confirm = useConfirm();
@@ -281,7 +282,7 @@ export default function ChannelPage({
   const metaParts = [
     ch?.handle ? `@${ch.handle.replace(/^@/, "")}` : null,
     ch?.subscriber_count != null
-      ? t("channel.subscribers", { formatted: formatViews(ch.subscriber_count) })
+      ? t("channel.subscribers", { formatted: formatCompact(ch.subscriber_count, lang) })
       : null,
     // Show "stored / total" when we only hold a subset of a subscribed channel's uploads, so the
     // gap between the browsable videos and YouTube's raw count is honest rather than looking like
@@ -292,7 +293,7 @@ export default function ChannelPage({
         : t("channel.videoCount", { count: ch.video_count })
       : null,
     ch?.total_view_count != null
-      ? t("channel.totalViews", { formatted: formatViews(ch.total_view_count) })
+      ? t("channel.totalViews", { formatted: formatCompact(ch.total_view_count, lang) })
       : null,
     // Per-type breakdown (R14 moved this off the manager table's N/S/L column). Only meaningful
     // once we actually hold some of the channel's videos.

frontend/src/components/ChannelStatsPopover.dom.test.tsx

@@ -14,12 +14,16 @@ import { FloatingTree } from "@floating-ui/react";
 // (i18n/index.ts calls i18n.use(initReactI18next) at import time).
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 import ChannelStatsPopover from "./ChannelStatsPopover";
 import { type MyStatus } from "../lib/api";
 import { formatEta, formatNumber } from "../lib/format";
+import type { TFunction } from "i18next";
 
 beforeAll(() => {
   class Stub {
@@ -74,7 +78,7 @@ describe("ChannelStatsPopover", () => {
     expect(screen.getByText("channels.stats.recentSynced")).toBeTruthy();
     expect(screen.getByText("3/10")).toBeTruthy(); // recent-synced ratio
     expect(screen.getByText("1/4")).toBeTruthy(); // full-history ratio
-    expect(screen.getByText(formatNumber(1234))).toBeTruthy(); // my_videos, locale-agnostic
+    expect(screen.getByText(formatNumber(1234, "en"))).toBeTruthy(); // my_videos, locale-agnostic
   });
 
   it("shows the 'left' ETA row only while a backfill is pending, with the formatted ETA value", () => {
@@ -86,7 +90,9 @@ describe("ChannelStatsPopover", () => {
     // The value is the FORMATTED ETA (via formatEta), not raw seconds — asserting it guards the
     // formatEta wiring, which is untested anywhere else. Using the same formatEta keeps this
     // locale/impl-agnostic (expected === what the component renders).
-    expect(screen.getByText(formatEta(120))).toBeTruthy();
+    expect(
+      screen.getByText(formatEta(((k: string) => k) as unknown as TFunction, 120)),
+    ).toBeTruthy();
     expect(screen.queryByText("120")).toBeNull(); // never the raw seconds
   });
 

frontend/src/components/ChannelStatsPopover.tsx

@@ -22,7 +22,8 @@ import { SURFACE } from "./ui/surfaces";
 // with a leading icon. Kept in the manager's own toolbar (the header's SyncStatus is global; these
 // are channel-catalog figures).
 export default function ChannelStatsPopover({ status }: { status: MyStatus }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const [open, setOpen] = useState(false);
   const pop = useAnchoredPopover({
     open,
@@ -58,7 +59,7 @@ export default function ChannelStatsPopover({ status }: { status: MyStatus }) {
           {
             icon: Clock,
             label: t("channels.stats.left"),
-            value: formatEta(status.deep_eta_seconds),
+            value: formatEta(t, status.deep_eta_seconds),
             hint: t("channels.stats.leftHint", { count: status.deep_pending_count }),
           },
         ]
@@ -66,13 +67,13 @@ export default function ChannelStatsPopover({ status }: { status: MyStatus }) {
     {
       icon: Film,
       label: t("channels.stats.myVideos"),
-      value: formatNumber(status.my_videos),
+      value: formatNumber(status.my_videos, lang),
       hint: t("channels.stats.myVideosHint"),
     },
     {
       icon: Zap,
       label: t("channels.stats.quotaLeft"),
-      value: formatNumber(status.quota_remaining_today),
+      value: formatNumber(status.quota_remaining_today, lang),
       hint: t("channels.stats.quotaLeftHint"),
     },
   ];

frontend/src/components/Channels.tsx

@@ -107,7 +107,8 @@ export default function Channels() {
   const { openChannel: onViewChannel, setPage } = useNavigationActions();
   const feedFilters = useFeedFilters();
   const { setFilters: setFeedFilters } = useFeedFiltersActions();
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
 
   // Clicking a tag chip filters the feed by that tag and jumps there. Internalised (was an App
@@ -364,6 +365,7 @@ export default function Channels() {
 
   const columns = buildChannelColumns({
     t,
+    lang: i18n.language,
     prioOptions,
     userTags,
     bumpPriority,
@@ -471,7 +473,7 @@ export default function Channels() {
   // Cards have no column headers, so apply the shared column filters (sync + any funnel set in table
   // mode) here via the exported predicate — the same filterRows DataTable runs internally — then sort.
   const sortedChannels = isCards
-    ? sortRows(filterRows(syncedChannels, columns, filters), columns, sort)
+    ? sortRows(filterRows(syncedChannels, columns, filters), columns, sort, lang)
     : syncedChannels;
   const {
     paged: pagedCards,

frontend/src/components/DebugRecorder.tsx

@@ -372,7 +372,8 @@ const chipTone = (s: DebugSession): string => {
 
 /** The last sessions, newest first — reopening one opens its composer, with what was written. */
 function SessionHistory() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const q = useQuery({ queryKey: qk.debugSessions(), queryFn: api.debugSessions });
   const items = q.data?.items.filter((s) => s.ended_at) ?? [];
   if (q.isError && !q.data)
@@ -394,7 +395,7 @@ function SessionHistory() {
               key={s.id}
               className="flex flex-wrap items-center gap-x-3 gap-y-1 px-3 py-2 text-sm"
             >
-              <span className="tabular-nums">{formatDateTimeSec(s.started_at)}</span>
+              <span className="tabular-nums">{formatDateTimeSec(s.started_at, lang)}</span>
               <span className="min-w-0 flex-1 truncate text-muted">
                 {s.draft?.title || t(`settings.diagnostics.levels.${s.level}`)}
               </span>

frontend/src/components/DiagnosticsSettings.tsx

@@ -40,7 +40,8 @@ function useRemaining(until: string | null): { h: number; m: number } | null {
 // the server keeps it and for how long, and that it lapses; only its button raises the level.
 // Returning to Alap needs no consent and happens at once.
 export default function DiagnosticsSettings() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const me = useMe();
   const qc = useQueryClient();
   const state = me.diagnostics;
@@ -173,8 +174,8 @@ export default function DiagnosticsSettings() {
                   level: levelName(current),
                   time:
                     remaining.h > 0
-                      ? `${formatUnit(remaining.h, "hour")} ${formatUnit(remaining.m, "minute")}`
-                      : formatUnit(remaining.m, "minute"),
+                      ? `${formatUnit(remaining.h, "hour", lang)} ${formatUnit(remaining.m, "minute", lang)}`
+                      : formatUnit(remaining.m, "minute", lang),
                 })}
               </span>
             </span>

frontend/src/components/DownloadCenter.confirm.dom.test.tsx

@@ -15,7 +15,10 @@ const mocks = vi.hoisted(() => ({
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 vi.mock("./confirmContext", () => ({ useConfirm: () => mocks.confirm }));
 vi.mock("../lib/useLiveQuery", () => ({

frontend/src/components/DownloadCenter.menu.dom.test.tsx

@@ -18,7 +18,10 @@ const mocks = vi.hoisted(() => ({
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 vi.mock("./confirmContext", () => ({ useConfirm: () => mocks.confirm }));
 vi.mock("../lib/useLiveQuery", () => ({

frontend/src/components/Feed.tsx

@@ -136,7 +136,8 @@ export default function Feed({
   // that are constant within one channel (subscribers, priority) — they'd be meaningless.
   channelScoped?: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n: ui } = useTranslation();
+  const lang = ui.language;
   // "Connect YouTube" onboarding lives in WizardProvider (the empty-state CTA when not connected).
   const { openWizard: onOpenWizard } = useWizard();
   // Same list the panel's badge counts — see lib/useActiveFilters.
@@ -439,6 +440,7 @@ export default function Feed({
       withOverrides(ytLoaded),
       ytSort.key as SortKey,
       ytSort.dir,
+      lang,
     );
     const ytPlayerQueue: Video[] = ytItems.filter((v) => v.status !== "hidden");
     const ytError =

frontend/src/components/Home.dom.test.tsx

@@ -31,12 +31,14 @@ beforeAll(() => {
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
+  // the real instance: these tests switch the language with setLanguage
+  const real = (await import("i18next")).default;
   return {
     ...actual,
     useTranslation: () => ({
       t: (key: string, opts?: Record<string, unknown>) =>
         opts?.time ? `${key}:${opts.time}` : key,
-      i18n: { language: "en" },
+      i18n: real,
     }),
   };
 });

frontend/src/components/Home.tsx

@@ -30,7 +30,13 @@ import {
 } from "../lib/api";
 import { notifyActionError } from "../lib/actionErrors";
 import { takeSurprise, useSurpriseRequest } from "../lib/homeIntent";
-import { formatDuration, formatRuntime, formatViews, relativeTime } from "../lib/format";
+import {
+  formatCompact,
+  formatDecimal,
+  formatDuration,
+  formatRuntime,
+  relativeTime,
+} from "../lib/format";
 import { toggleWatchState } from "../lib/plexWatch";
 import { useComfortPin, pinTargetForCard, shelfMoveTarget, type ShelfMove } from "./comfortPin";
 import { type ActionMenuItem } from "./ui/ActionMenu";
@@ -502,7 +508,8 @@ function Hero({
   onReroll: () => void;
   rerolling: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // The hero is a card like any other (the owner, 2026-09-27): the same hover row on its picture and
   // the same whole action list on its "…", right-click and long-press — it was a pin row alone.
   const { menu, items, panels, plexAdd } = useHomeCardMenu({
@@ -533,12 +540,12 @@ function Hero({
   const plex = card.plex;
   const video = card.video;
   const facts = plex
-    ? [plex.year, formatRuntime(card.duration_seconds)]
+    ? [plex.year, formatRuntime(card.duration_seconds, lang)]
     : video
       ? // The channel is the subtitle already.
         [
-          video.view_count != null && `${formatViews(video.view_count)} ${t("card.views")}`,
-          relativeTime(video.published_at),
+          video.view_count != null && `${formatCompact(video.view_count, lang)} ${t("card.views")}`,
+          relativeTime(video.published_at, lang),
         ]
       : [];
   const factLine = facts.filter(Boolean).join(" · ");
@@ -631,7 +638,7 @@ function Hero({
               <span className="inline-flex items-center gap-0.5">
                 {factLine && <span aria-hidden>·</span>}
                 <Star className="h-3 w-3 fill-current text-rating" aria-hidden />
-                {plex.rating.toFixed(1)}
+                {formatDecimal(plex.rating, lang)}
               </span>
             )}
           </p>
@@ -692,7 +699,7 @@ function readSavedQuestion(): SavedQuestion | null {
 }
 
 export default function Home() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   const { setPage } = useNavigationActions();
   const { setJumpTo } = usePlexActions();
@@ -722,7 +729,7 @@ export default function Home() {
   const [player, setPlayer] = useState<{ video: Video; startAt: number | null } | null>(null);
 
   // A Plex card's title is in the reader's language (E30), so the answer is cached per language.
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const q = useQuery({
     queryKey: qk.discover(budget, lang),
     queryFn: () => api.discover(budget, 0, lang),

frontend/src/components/IssueComposer.tsx

@@ -114,6 +114,7 @@ function ComposerBody({
   total: number;
 }) {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const pageName = (page: string) => {
     const def = moduleDef(page as Page);
@@ -304,8 +305,8 @@ function ComposerBody({
     <div ref={root} className="flex flex-col gap-4">
       <p className="text-sm text-muted">
         {t("recorder.composer.meta", {
-          start: formatDateTimeSec(started.toISOString()),
-          minutes: formatUnit(minutes, "minute"),
+          start: formatDateTimeSec(started.toISOString(), lang),
+          minutes: formatUnit(minutes, "minute", lang),
           level: t(`settings.diagnostics.levels.${session.level}`),
         })}
       </p>
@@ -607,7 +608,8 @@ function Attachments({
   onReview: (source: Source) => void;
   reviewed: MutableRefObject<Source | null>;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const shots = shotsQ.data?.items ?? [];
   // A removed screenshot's ✕ goes with it: the focus moves to the file picker, which stays.
@@ -685,7 +687,7 @@ function Attachments({
           />
           <div className="flex min-w-0 flex-1 flex-col">
             <span className="text-sm">{t("recorder.composer.screenshot", { n: i + 1 })}</span>
-            <span className="text-xs text-muted tabular-nums">{formatBytes(shot.size)}</span>
+            <span className="text-xs text-muted tabular-nums">{formatBytes(shot.size, lang)}</span>
           </div>
           {!readOnly && (
             <IconButton

frontend/src/components/Messages.keyboard.dom.test.tsx

@@ -8,7 +8,10 @@ import { act, cleanup, render, screen } from "@testing-library/react";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 const conversation = (id: number, name: string) => ({

frontend/src/components/NotificationsPanel.menu.dom.test.tsx

@@ -23,7 +23,7 @@ const h = vi.hoisted(() => ({
 
 vi.mock("react-i18next", async (importOriginal) => ({
   ...(await importOriginal<typeof import("react-i18next")>()),
-  useTranslation: () => ({ t: (key: string) => key }),
+  useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
 }));
 vi.mock("./confirmContext", () => ({ useConfirm: () => h.confirm }));
 vi.mock("../lib/useLiveQuery", () => ({

frontend/src/components/NotificationsPanel.tsx

@@ -507,6 +507,7 @@ function NotificationRow({
   onOpenScheduler: () => void;
   onOpenDownloads: () => void;
 }) {
+  const lang = useTranslation().i18n.language;
   // The maintenance producer ships the affected videos in `data.videos`; list a few titles.
   const videos: { id: string; title: string | null }[] = Array.isArray(n.data?.videos)
     ? n.data!.videos
@@ -535,7 +536,7 @@ function NotificationRow({
       job,
     });
     const result = d.result ?? parseSummary(d.summary);
-    body = result ? jobResultText(t, result) : d.summary || n.body;
+    body = result ? jobResultText(t, result, lang) : d.summary || n.body;
   } else if (isEmailUndelivered) {
     // Email is fail-soft: a drop leaves no error anywhere else, so this notice IS the trace. Name
     // the message that was lost, since that is what tells the operator what their users didn't get.

frontend/src/components/PersonalViewsModal.dom.test.tsx

@@ -22,6 +22,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   return {
     ...actual,
     useTranslation: () => ({
+      i18n: { language: "en" },
       t: (key: string, o?: { name?: string }) =>
         key === "common.order.dragNamed" ? `${key}:${o?.name}` : key,
     }),

frontend/src/components/PlayerModal.tsx

@@ -44,7 +44,7 @@ import {
   channelYouTubeUrl,
   formatDate,
   formatDuration,
-  formatViews,
+  formatCompact,
   relativeTime,
 } from "../lib/format";
 import { renderDescription } from "./descriptionLinks";
@@ -119,6 +119,7 @@ export default function PlayerModal({
   // icon next to the name keeps the open-on-YouTube behaviour.
 }) {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const { openChannel } = useNavigationActions();
   const qc = useQueryClient();
   const descFade = useScrollFade();
@@ -899,12 +900,12 @@ export default function PlayerModal({
                       ·{" "}
                       {t("player.views", {
                         count: active.view_count,
-                        formattedCount: formatViews(active.view_count),
+                        formattedCount: formatCompact(active.view_count, lang),
                       })}
                     </span>
                   )}
                   <span>
-                    · {relativeTime(active.published_at)}
+                    · {relativeTime(active.published_at, lang)}
                     {active.published_at && ` · ${fullDate(active.published_at)}`}
                   </span>
                   {active.duration_seconds != null && (
@@ -924,13 +925,13 @@ export default function PlayerModal({
                       ·{" "}
                       {t("player.views", {
                         count: detail.data.view_count,
-                        formattedCount: formatViews(detail.data.view_count),
+                        formattedCount: formatCompact(detail.data.view_count, lang),
                       })}
                     </span>
                   )}
                   {detail.data?.published_at && (
                     <span>
-                      · {relativeTime(detail.data.published_at)} ·{" "}
+                      · {relativeTime(detail.data.published_at, lang)} ·{" "}
                       {fullDate(detail.data.published_at)}
                     </span>
                   )}

frontend/src/components/Playlists.tsx

@@ -160,7 +160,8 @@ export default function Playlists() {
   // The selected playlist lives in PlaylistsProvider, shared with the PlaylistsRail (which owns the
   // list + its rail controls). This detail pane reacts to the selection.
   const { selectedId } = usePlaylists();
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const plActions = usePlaylistActions();
   const [renaming, setRenaming] = useState(false);
@@ -391,8 +392,8 @@ export default function Playlists() {
 
   const filterActive = Object.values(filters).some(isActive);
   const shown = useMemo(
-    () => applyView(filterRows(items, columns, filters), columns, viewSort, groupBy),
-    [items, columns, filters, viewSort, groupBy],
+    () => applyView(filterRows(items, columns, filters), columns, viewSort, groupBy, lang),
+    [items, columns, filters, viewSort, groupBy, lang],
   );
   // Read by the drag announcements, which must report the CURRENT positions without making the
   // announcement closures a render dependency of DndContext.
@@ -909,7 +910,7 @@ export default function Playlists() {
                 <div className="flex items-center gap-2 mt-1 flex-wrap">
                   <span className="text-xs text-muted">
                     {t("playlists.itemCount", { count: items.length })}
-                    {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds)}`}
+                    {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds, lang)}`}
                     {filterActive && ` · ${t("playlists.filteredCount", { count: shown.length })}`}
                   </span>
                   {(items.length > 1 || viewActive) && (

frontend/src/components/PlaylistsRail.tsx

@@ -4,6 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { ListVideo, Pin, Plus, RefreshCw, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
+import { compareText } from "../lib/collate";
 import { formatRuntime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -59,7 +60,8 @@ export default function PlaylistsRail({
   // box and the Playlists detail pane); this rail owns the list query + its own controls.
   const { search, selectedId } = usePlaylists();
   const { setSelectedId } = usePlaylistsActions();
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [plSort, setPlSort] = useState<PlSort>(() => readAccountMerged(LS.plSort, PL_SORT_DEFAULT));
   const plSortOptions: SortOption[] = [
@@ -123,7 +125,7 @@ export default function PlaylistsRail({
       if (plSort.dirtyFirst && a.dirty !== b.dirty) return a.dirty ? -1 : 1;
       switch (plSort.key) {
         case "name":
-          return sign * plName(a).localeCompare(plName(b));
+          return sign * compareText(plName(a), plName(b), lang);
         case "count":
           return sign * (a.item_count - b.item_count);
         case "duration":
@@ -134,7 +136,7 @@ export default function PlaylistsRail({
           return 0;
       }
     });
-  }, [playlists, plSort, plName]);
+  }, [playlists, plSort, plName, lang]);
 
   const q = search.trim().toLowerCase();
   const visiblePlaylists = useMemo(
@@ -366,7 +368,7 @@ export default function PlaylistsRail({
                       <div className="text-2xs text-muted">
                         {t("playlists.itemCount", { count: pl.item_count })}
                         {pl.total_duration_seconds > 0 &&
-                          ` · ${formatRuntime(pl.total_duration_seconds)}`}
+                          ` · ${formatRuntime(pl.total_duration_seconds, lang)}`}
                       </div>
                     </div>
                   </Pressable>

frontend/src/components/PlexBrowse.tsx

@@ -93,7 +93,7 @@ const BACKABLE_VIEWS: readonly string[] = [
 ];
 
 export default function PlexBrowse() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   // Scope / watch-state / sort / filters live in PlexProvider (edited mainly from PlexSidebar); the
   // search term (q) comes from the shared header box; the rail hands off a playlist to open via the
@@ -103,7 +103,7 @@ export default function PlexBrowse() {
   const dq = useDebounced(q.trim(), 350);
   // Active language for the HU overlay: "hu" serves Hungarian metadata, else EN. In the query keys so
   // a switch refetches the grid/facets/detail; useTranslation re-renders this component on switch.
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   // The language the grid query is allowed to run at — lags `lang` by one render on a switch so the
   // genre-clear effect (below) can settle first (see the effect for why).
   const [committedLang, setCommittedLang] = useState(lang);
@@ -116,7 +116,7 @@ export default function PlexBrowse() {
   // The provider holds sort scope-agnostically; heal it against THIS scope's options so a movie-only
   // key (e.g. duration) left over from before a switch to shows doesn't ask for an unavailable sort.
   const sortState = healPlexSort(sort, scope);
-  const sortOptions = plexSortOptions(scope, (k) => t(`plex.sort.${k}`));
+  const sortOptions = plexSortOptions(scope, (k) => t(`plex.sort.${k}`), lang);
   // Same idiom for the search scope-pill: a movie-only scope can't hold the Episode field.
   const fieldState = healPlexField(field, scope);
   // Discover is the landing (E5 S4); the flat grid is the "Browse all" secondary view. A header search

frontend/src/components/PlexCollectionView.dom.test.tsx

@@ -27,7 +27,12 @@ beforeAll(() => {
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  // the real instance: these tests switch the language with setLanguage
+  const real = (await import("i18next")).default;
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: real, t: (key: string) => key }),
+  };
 });
 vi.mock("./confirmContext", () => ({ useConfirm: () => async () => true }));
 

frontend/src/components/PlexCollectionView.tsx

@@ -67,7 +67,7 @@ export default function PlexCollectionView({
   onRestartCard: (card: PlexCard) => void;
   onInfoCard: (card: PlexCard) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
   // One menu for the member grid (see ui/ContextMenu).
@@ -84,7 +84,7 @@ export default function PlexCollectionView({
   // ONE key for the read and every write into it: when the sort joined the key, the watched toggle's
   // optimistic write kept the old spelling and landed on an entry nothing reads (setQueryData matches
   // exactly), so the click did nothing visible until a reload.
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const detailKey = qk.plexCollections("detail", collectionId, sortPref.key, sortPref.dir, lang);
   const q = useQuery({
     queryKey: detailKey,
@@ -118,7 +118,7 @@ export default function PlexCollectionView({
   // what the add-panel searches and which sort keys are on offer (only a movie has a runtime).
   const scope = items.some((c) => c.type === "show") ? "show" : "movie";
   const sortState = healPlexSort(sortPref, scope);
-  const sortOptions = plexSortOptions(scope, (k) => t(`plex.sort.${k}`));
+  const sortOptions = plexSortOptions(scope, (k) => t(`plex.sort.${k}`), lang);
   // Movies/episodes are playable leaves that queue as themselves; a show can't queue directly, so
   // Play-all expands it into its episodes (buildQueue). Either kind means there's something to play —
   // this is what a show-collection needs: before, `playable` held only leaves, so a show-collection's
@@ -264,7 +264,7 @@ export default function PlexCollectionView({
       (d.year_max && d.year_max !== d.year_min ? `${d.year_min} — ${d.year_max}` : `${d.year_min}`),
     d?.content_rating,
     d && t("plex.browse.items", { count: d.child_count ?? items.length }),
-    formatRuntime(d?.total_runtime_s),
+    formatRuntime(d?.total_runtime_s, lang),
   ].filter(Boolean) as string[];
 
   return (

frontend/src/components/PlexDiscover.dom.test.tsx

@@ -27,6 +27,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   return {
     ...actual,
     useTranslation: () => ({
+      i18n: { language: "en" },
       t: (key: string, opts?: Record<string, unknown>) =>
         opts?.title ? `${key}:${opts.title}` : key,
     }),

frontend/src/components/PlexDiscover.tsx

@@ -108,9 +108,9 @@ export default function PlexDiscover({
   onGenre: (genre: string) => void;
   onSeeAllRecent: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   // The active sidebar filters narrow every rail, so Discover matches the Browse grid. They're in the
   // query keys so a filter change refetches (and the watched-toggle setQueryData below keys the same).
   const { filters } = usePlex();

frontend/src/components/PlexInfo.dom.test.tsx

@@ -14,6 +14,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   return {
     ...actual,
     useTranslation: () => ({
+      i18n: { language: "en" },
       t: (key: string, opts?: Record<string, unknown>) =>
         opts?.name ? `${key}:${opts.name}` : key,
     }),

frontend/src/components/PlexInfo.tsx

@@ -4,12 +4,11 @@ import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
 import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } from "lucide-react";
 import { api, type PlexCard, type PlexFilters, type PlexItemDetail } from "../lib/api";
-import { formatRuntime } from "../lib/format";
+import { formatCompact, formatDecimal, formatMoney, formatRuntime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
 import { DetailCustomizeMenu, Filterable, PrefToggle } from "./plexDetailUi";
 import { tmdbBackdropUrl, useCrossfadeBackdrop, useDetailPrefs } from "../lib/plexDetail";
 import { usePref } from "../lib/usePref";
-import i18n from "../i18n";
 import { Carousel } from "./ui/Carousel";
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import PlexPlaylistAdd from "./PlexPlaylistAdd";
@@ -38,24 +37,6 @@ import { regionName, vocab } from "../lib/vocab";
 // The TMDB-enriched pieces (multi-source ratings row, facts, keyword chips, full crew — E5 S5) render
 // on the page variant only; the overlay stays lean.
 
-// Localize the grouping/compact notation to the active language (the app convention, lib/api/core.ts);
-// `i18n.language` is read at call time, and the component re-renders on a language switch. Currency stays
-// USD — TMDB budget/revenue figures are USD regardless of the viewer's language.
-const compact = (n: number) =>
-  new Intl.NumberFormat(i18n.language, { notation: "compact" }).format(n);
-const money = (n: number) =>
-  new Intl.NumberFormat(i18n.language, {
-    notation: "compact",
-    style: "currency",
-    currency: "USD",
-    // The minimum is load-bearing, not a default written out: given only a maximum, a compact
-    // format is free to pad to it. The very same options render "$160.0M" on Node 22 and "$160M"
-    // on Node 24 (measured 2026-09-20 — the server gate was red on exactly this while the local one
-    // was green), and browsers split along the same line. Pin it rather than inherit the engine's.
-    minimumFractionDigits: 0,
-    maximumFractionDigits: 1,
-  }).format(n);
-
 // One rating chip's number: a filter button when this is the score the backend's ratingMin column
 // tracks, plain text otherwise. It lives at MODULE level, not inside RatingsRow, because a component
 // declared during render is a NEW component type on every render — React unmounts and remounts the
@@ -70,7 +51,8 @@ function Score({
   filterable: boolean;
   onFilterRating?: (score: number) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const shown = formatDecimal(score, i18n.language);
   return filterable && onFilterRating ? (
     <Pressable
       onClick={() => onFilterRating(Math.floor(score))}
@@ -78,12 +60,12 @@ function Score({
       title={t("plex.info.filterRating", { n: Math.floor(score) })}
     >
       <Star className="h-3.5 w-3.5 fill-current" />
-      {score.toFixed(1)}
+      {shown}
     </Pressable>
   ) : (
     <span className="inline-flex items-center gap-1">
       <Star className="h-3.5 w-3.5 fill-current" />
-      {score.toFixed(1)}
+      {shown}
     </span>
   );
 }
@@ -110,7 +92,7 @@ function RatingsRow({
    *  theme's dark gold would sit on the player's black (2.6:1, measured). */
   onDark?: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const primary = imdbRating ?? audience ?? null; // the filterable score
   // The source label is the chip's own colour at full strength: dimmed to 70% it fell under 4.5:1.
   const chipCls = `inline-flex items-center gap-1 rounded-md px-1.5 py-0.5 font-semibold ${
@@ -147,7 +129,7 @@ function RatingsRow({
         <span className={chipCls}>
           <Score score={tmdbRating} filterable={false} />
           <span className="text-2xs font-normal">
-            TMDb{has(tmdbVotes) ? ` · ${compact(tmdbVotes)}` : ""}
+            TMDb{has(tmdbVotes) ? ` · ${formatCompact(tmdbVotes, i18n.language)}` : ""}
           </span>
         </span>
       )}
@@ -171,7 +153,8 @@ function FactsPanel({
   detail: PlexItemDetail;
   onFilter?: (patch: Partial<PlexFilters>) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const tmdb = detail.tmdb;
   const studios = tmdb?.studios?.length ? tmdb.studios : detail.studio ? [detail.studio] : [];
   const rows: { key: string; label: string; node: ReactNode }[] = [];
@@ -195,7 +178,7 @@ function FactsPanel({
     rows.push({
       key: "runtime",
       label: t("plex.info.facts.runtime"),
-      node: formatRuntime(detail.duration_seconds),
+      node: formatRuntime(detail.duration_seconds, lang),
     });
   const cert = detail.content_rating;
   if (cert)
@@ -209,9 +192,17 @@ function FactsPanel({
       ),
     });
   if (tmdb?.budget)
-    rows.push({ key: "budget", label: t("plex.info.facts.budget"), node: money(tmdb.budget) });
+    rows.push({
+      key: "budget",
+      label: t("plex.info.facts.budget"),
+      node: formatMoney(tmdb.budget, lang),
+    });
   if (tmdb?.revenue)
-    rows.push({ key: "revenue", label: t("plex.info.facts.revenue"), node: money(tmdb.revenue) });
+    rows.push({
+      key: "revenue",
+      label: t("plex.info.facts.revenue"),
+      node: formatMoney(tmdb.revenue, lang),
+    });
   if (studios.length)
     rows.push({
       key: "studios",
@@ -372,7 +363,8 @@ export default function PlexInfo({
   onPlayItem,
   library,
 }: Props) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
 
   // Personal display prefs (default ON), shared with the series detail pages via useDetailPrefs:
@@ -601,7 +593,7 @@ export default function PlexInfo({
                   </Filterable>
                 )}
                 {detail.duration_seconds ? (
-                  <span className={mutedCls}>{formatRuntime(detail.duration_seconds)}</span>
+                  <span className={mutedCls}>{formatRuntime(detail.duration_seconds, lang)}</span>
                 ) : null}
                 {detail.studio && (
                   <Filterable

frontend/src/components/PlexPersonView.dom.test.tsx

@@ -25,7 +25,12 @@ beforeAll(() => {
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  // the real instance: these tests switch the language with setLanguage
+  const real = (await import("i18next")).default;
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: real, t: (key: string) => key }),
+  };
 });
 
 const plexPerson = vi.fn();
@@ -116,7 +121,8 @@ describe("PlexPersonView", () => {
     // E30: TMDB's English department, named through the vocab (key-echo i18n here).
     expect(screen.getByText("vocab.department.Acting")).toBeTruthy();
     expect(screen.getByText("A celebrated actor.")).toBeTruthy();
-    expect(screen.getByText(/1970-01-01/)).toBeTruthy();
+    // E30 S3: the birth date as a date in the UI language, not the raw ISO string
+    expect(screen.getByText("Jan 1, 1970 · Springfield")).toBeTruthy();
     expect(screen.getByText(/plex\.person\.aka/)).toBeTruthy();
     expect(screen.getByText(/Janie D/)).toBeTruthy();
     // "Known for" rail renders its card.

frontend/src/components/PlexPlayer.tsx

@@ -145,7 +145,7 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
   const qc = useQueryClient();
   const [id, setId] = useState(itemId);
   // Serve the player's title/summary/rich detail in the active language (E5 S6), matching the grid.
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const detailQ = useQuery({
     queryKey: qk.plexItem(id, lang),
     queryFn: () => api.plexItem(id, lang),
@@ -986,7 +986,7 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
         seekTo(playhead(absRef, loadStartRef) + zone * TAP_SKIP_S);
         flashOsd(
           t(zone < 0 ? "plex.player.tapBack" : "plex.player.tapForward", {
-            d: formatUnit(skips * TAP_SKIP_S, "second"),
+            d: formatUnit(skips * TAP_SKIP_S, "second", lang),
           }),
         );
         return;
@@ -999,7 +999,7 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
       }
       toggleControls();
     },
-    [togglePlay, showControls, toggleControls, seekTo, flashOsd, t, uiVisible],
+    [togglePlay, showControls, toggleControls, seekTo, flashOsd, t, lang, uiVisible],
   );
 
   // Auto-skip intro/credits (countdown → jump to marker end / next item). The refs bridge to the

frontend/src/components/PlexPlayerSettings.dom.test.tsx

@@ -10,7 +10,7 @@ import { cleanup, fireEvent, render, screen } from "@testing-library/react";
 
 vi.mock("react-i18next", async (importOriginal) => ({
   ...(await importOriginal<typeof import("react-i18next")>()),
-  useTranslation: () => ({ t: (k: string) => k }),
+  useTranslation: () => ({ i18n: { language: "en" }, t: (k: string) => k }),
 }));
 
 import { PlexPlayerSettings } from "./PlexPlayerSettings";

frontend/src/components/PlexPlayerSettings.tsx

@@ -84,7 +84,7 @@ export function PlexPlayerSettings(props: {
               min={-10}
               max={10}
               step={0.1}
-              suffix="s"
+              seconds
               onChange={(v) => patchPrefs({ subOffset: v })}
               onReset={() => patchPrefs({ subOffset: 0 })}
             />
@@ -94,7 +94,7 @@ export function PlexPlayerSettings(props: {
               min={-5}
               max={5}
               step={0.1}
-              suffix="s"
+              seconds
               onChange={changeAudioOffset}
               onReset={() => changeAudioOffset(0)}
             />
@@ -109,7 +109,7 @@ export function PlexPlayerSettings(props: {
               min={5}
               max={30}
               step={1}
-              suffix="s"
+              seconds
               onChange={(v) => patchPrefs({ seekStep: v })}
             />
             <Slider
@@ -118,7 +118,7 @@ export function PlexPlayerSettings(props: {
               min={0.1}
               max={5}
               step={0.1}
-              suffix="s"
+              seconds
               onChange={(v) => patchPrefs({ fineSeekStep: v })}
             />
             <Toggle
@@ -148,7 +148,7 @@ export function PlexPlayerSettings(props: {
                 min={0}
                 max={10}
                 step={1}
-                suffix="s"
+                seconds
                 onChange={(v) => patchPrefs({ autoSkipDelay: v })}
               />
             )}

frontend/src/components/PlexPlaylistView.menu.dom.test.tsx

@@ -9,13 +9,16 @@ import { describe, it, expect, vi, beforeAll, afterEach } from "vitest";
 import { render, cleanup, screen, fireEvent, waitFor, act } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
-vi.mock("react-i18next", async (importOriginal) => ({
-  ...(await importOriginal<typeof import("react-i18next")>()),
-  useTranslation: () => ({
-    t: (key: string, o?: { title?: string }) => (o?.title ? `${key}:${o.title}` : key),
-    i18n: { language: "en" },
-  }),
-}));
+vi.mock("react-i18next", async (importOriginal) => {
+  const real = (await import("i18next")).default;
+  return {
+    ...(await importOriginal<typeof import("react-i18next")>()),
+    useTranslation: () => ({
+      t: (key: string, o?: { title?: string }) => (o?.title ? `${key}:${o.title}` : key),
+      i18n: real,
+    }),
+  };
+});
 const h = vi.hoisted(() => ({ notify: vi.fn(), confirm: vi.fn(async () => true) }));
 vi.mock("../lib/notifications", async (importOriginal) => ({
   ...(await importOriginal<typeof import("../lib/notifications")>()),

frontend/src/components/PlexPlaylistView.tsx

@@ -406,7 +406,8 @@ export default function PlexPlaylistView({
   /** A title's media-info page, from its menu's "Details". */
   onInfoCard?: (card: PlexCard) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   const [renaming, setRenaming] = useState(false);
@@ -755,7 +756,7 @@ export default function PlexPlaylistView({
             {items.length > 0 && (
               <div className="mt-0.5 text-sm text-muted">
                 {t("plex.browse.items", { count: items.length })}
-                {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds)}`}
+                {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds, lang)}`}
               </div>
             )}
           </div>

frontend/src/components/PlexSidebar.tsx

@@ -44,7 +44,7 @@ const DURATION_BUCKETS: { key: string; min: number | null; max: number | null }[
 ];
 
 export default function PlexSidebar({ layout, setLayout, collapsed, onToggleCollapse }: Props) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   // Scope / watch-state / sort / expanded filters live in PlexProvider (shared with PlexBrowse and
   // the header search box); this rail is the primary editor of them. Opening a playlist is a
   // hand-off to PlexBrowse via the shared `playlistOpen` slot.
@@ -60,7 +60,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   // Titles are display-only (chip labels) — keep them OUT of the facet query key so renaming/relabelling
   // never refetches. collectionMode stays IN the key: any/all changes the result set.
   const { collectionTitles: _cts, ...facetKey } = filters;
-  const lang = plexLang(); // HU/EN genre vocabulary — refetch facets on a language switch
+  const lang = plexLang(i18n.language); // HU/EN genre vocabulary — refetch facets on a language switch
   const facetsQ = useQuery({
     queryKey: qk.plexFacets(scope, show, facetKey, lang),
     queryFn: () => api.plexFacets(scope, filters, show, lang),

frontend/src/components/PlexStatsOverlay.tsx

@@ -38,7 +38,8 @@ export function PlexStatsOverlay({
   durationRef: RefObject<number>;
   onClose: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const [rows, setRows] = useState<StatRow[]>([]);
 
   useEffect(() => {
@@ -87,6 +88,7 @@ export function PlexStatsOverlay({
 
       setRows(
         buildStatRows({
+          lang,
           playable,
           hwEncoder: hwRef.current,
           tonemap: tonemapRef.current,
@@ -110,6 +112,7 @@ export function PlexStatsOverlay({
     const iv = setInterval(sample, 1000);
     return () => clearInterval(iv);
   }, [
+    lang,
     videoRef,
     hlsRef,
     fragMeterRef,

frontend/src/components/Scheduler.dom.test.tsx

@@ -11,7 +11,7 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (k: string) => k }) };
+  return { ...actual, useTranslation: () => ({ i18n: { language: "en" }, t: (k: string) => k }) };
 });
 
 import Scheduler from "./Scheduler";

frontend/src/components/Scheduler.menu.dom.test.tsx

@@ -9,7 +9,7 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 const h = vi.hoisted(() => ({ confirm: vi.fn(async () => true) }));
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (k: string) => k }) };
+  return { ...actual, useTranslation: () => ({ i18n: { language: "en" }, t: (k: string) => k }) };
 });
 vi.mock("./confirmContext", () => ({ useConfirm: () => h.confirm }));
 

frontend/src/components/Scheduler.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useId, useRef, useState } from "react";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
-import i18n from "../i18n";
+import type { TFunction } from "i18next";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import {
   Activity,
@@ -47,14 +47,14 @@ function secsUntil(iso: string | null): number | null {
   return Math.round((new Date(iso).getTime() - Date.now()) / 1000);
 }
 
-function fmtCountdown(s: number): string {
-  if (s <= 0) return i18n.t("time.eta.now");
+function fmtCountdown(t: TFunction, s: number, lang: string): string {
+  if (s <= 0) return t("time.eta.now");
   // The countdown speaks the UI language's units (DESIGN Copy), like every other duration.
-  if (s < 60) return formatUnit(s, "second");
+  if (s < 60) return formatUnit(s, "second", lang);
   const m = Math.floor(s / 60);
-  if (m < 60) return `${formatUnit(m, "minute")} ${formatUnit(s % 60, "second")}`;
+  if (m < 60) return `${formatUnit(m, "minute", lang)} ${formatUnit(s % 60, "second", lang)}`;
   const h = Math.floor(m / 60);
-  return `${formatUnit(h, "hour")} ${formatUnit(m % 60, "minute")}`;
+  return `${formatUnit(h, "hour", lang)} ${formatUnit(m % 60, "minute", lang)}`;
 }
 
 const DOT_CLASS: Record<string, string> = {
@@ -104,7 +104,8 @@ function StatusLegend() {
 // label and an indeterminate sliver — so every active run is visible, not just the ones
 // that report numbers.
 function JobProgress({ p }: { p: SchedulerJob["progress"] }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const phase = p?.phase ? t(`scheduler.phase.${p.phase}`, p.phase) : t("scheduler.phase.working");
   const pct =
     p?.total && p.total > 0 ? Math.min(100, Math.round((p.current / p.total) * 100)) : null;
@@ -114,9 +115,9 @@ function JobProgress({ p }: { p: SchedulerJob["progress"] }) {
         <span className="truncate">{phase}</span>
         <span className="tabular-nums shrink-0">
           {p?.total != null
-            ? `${formatNumber(p.current)} / ${formatNumber(p.total)}`
+            ? `${formatNumber(p.current, lang)} / ${formatNumber(p.total, lang)}`
             : p
-              ? formatNumber(p.current)
+              ? formatNumber(p.current, lang)
               : ""}
         </span>
       </div>
@@ -152,7 +153,8 @@ function JobRow({
   /** A run-all in flight: transient, natively disabled. */
   runBusy: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // The box shows the server's value and holds a draft only while it is being edited, so there is
   // nothing for an effect to re-sync: `null` means "follow the job". The poll behind this page
   // refreshes every few seconds, which is exactly when a mirror-plus-effect would have shown the
@@ -247,17 +249,17 @@ function JobRow({
         </div>
         <div
           className="text-2xs text-muted truncate"
-          title={(!job.running && formatDateTime(job.last_finished)) || undefined}
+          title={(!job.running && formatDateTime(job.last_finished, lang)) || undefined}
         >
           {job.running
             ? t("scheduler.runningNow")
             : job.last_finished
-              ? t("scheduler.lastRun", { time: relativeTime(job.last_finished) })
+              ? t("scheduler.lastRun", { time: relativeTime(job.last_finished, lang) })
               : t("scheduler.neverRun")}
           {job.last_error ? (
             <span className="text-danger"> · {job.last_error}</span>
           ) : job.result ? (
-            <span> · {jobResultText(t, job.result)}</span>
+            <span> · {jobResultText(t, job.result, lang)}</span>
           ) : job.last_result ? (
             <span> · {job.last_result}</span>
           ) : null}
@@ -270,7 +272,7 @@ function JobRow({
         ) : next != null ? (
           <>
             <div className="uppercase tracking-wide text-3xs">{t("scheduler.nextRun")}</div>
-            <div>{fmtCountdown(next)}</div>
+            <div>{fmtCountdown(t, next, lang)}</div>
           </>
         ) : (
           "—"
@@ -303,7 +305,8 @@ function MaintenanceSettings({
   onSave: (batch: number) => void;
   saving: boolean;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // Same shape as the job row above: a draft while editing, the server's value otherwise. This is
   // the editor a poll really could overwrite mid-keystroke before E27 S2 — its old effect had no
   // `editing` guard at all.
@@ -363,7 +366,7 @@ function MaintenanceSettings({
       )}
       <div className="text-2xs text-muted mt-1.5">
         {t("scheduler.maintenance.batchNote", {
-          default: formatNumber(m.revalidate_batch_default),
+          default: formatNumber(m.revalidate_batch_default, lang),
         })}
       </div>
     </div>
@@ -406,7 +409,8 @@ function Stat({
 }
 
 export default function Scheduler() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const confirm = useConfirm();
   // Poll faster while any job is running so live progress is smooth, then ease back when
@@ -647,31 +651,31 @@ export default function Scheduler() {
             icon={Database}
             label={t("scheduler.queue.recentPending")}
             hint={t("scheduler.queue.recentPendingHint")}
-            value={formatNumber(data.queue.channels_recent_pending)}
+            value={formatNumber(data.queue.channels_recent_pending, lang)}
           />
           <Stat
             icon={Database}
             label={t("scheduler.queue.deepPending")}
             hint={t("scheduler.queue.deepPendingHint")}
-            value={formatNumber(data.queue.channels_deep_pending)}
+            value={formatNumber(data.queue.channels_deep_pending, lang)}
           />
           <Stat
             icon={Clock}
             label={t("scheduler.queue.enrichPending")}
             hint={t("scheduler.queue.enrichPendingHint")}
-            value={formatNumber(data.queue.videos_pending_enrich)}
+            value={formatNumber(data.queue.videos_pending_enrich, lang)}
           />
           <Stat
             icon={Clock}
             label={t("scheduler.queue.shortsPending")}
             hint={t("scheduler.queue.shortsPendingHint")}
-            value={formatNumber(data.queue.videos_pending_shorts)}
+            value={formatNumber(data.queue.videos_pending_shorts, lang)}
           />
           <Stat
             icon={RadioTower}
             label={t("scheduler.queue.liveRefresh")}
             hint={t("scheduler.queue.liveRefreshHint")}
-            value={formatNumber(data.queue.videos_live_refresh)}
+            value={formatNumber(data.queue.videos_live_refresh, lang)}
           />
         </div>
       </div>
@@ -684,15 +688,15 @@ export default function Scheduler() {
             <Gauge className="w-4 h-4 text-muted shrink-0" />
             <span>
               {t("scheduler.quotaUsed", {
-                used: formatNumber(data.quota.used_today),
-                budget: formatNumber(data.quota.daily_budget),
+                used: formatNumber(data.quota.used_today, lang),
+                budget: formatNumber(data.quota.daily_budget, lang),
               })}
             </span>
             <span className="flex-1" />
             <span className="text-muted text-xs">
               {t("scheduler.quotaRemaining", {
                 count: data.quota.remaining_today,
-                formatted: formatNumber(data.quota.remaining_today),
+                formatted: formatNumber(data.quota.remaining_today, lang),
               })}
             </span>
           </div>

frontend/src/components/SettingsPanel.tsx

@@ -1,7 +1,7 @@
 import { useState } from "react";
 import { withActiveAccount } from "../lib/activeAccount";
 import { qk } from "../lib/queryKeys";
-import { formatDate, formatDateTime } from "../lib/format";
+import { formatDate, formatDateTime, formatUnit } from "../lib/format";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Activity, Bell, Check, Loader2, Monitor, RefreshCw, Trash2, User } from "lucide-react";
@@ -245,7 +245,8 @@ function Appearance({ prefs }: { prefs: PrefsController }) {
 }
 
 function Notifications({ prefs }: { prefs: PrefsController }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // Controlled by PrefsProvider (live preview via configureNotifications there), which auto-saves
   // it. The "send test" button below fires immediately.
   const { notif: cfg, setNotif } = prefs;
@@ -270,7 +271,9 @@ function Notifications({ prefs }: { prefs: PrefsController }) {
         <div className="py-1.5 text-sm">
           <div className="flex items-center justify-between">
             <span className="text-muted text-xs">{t("settings.notifications.dismissAfter")}</span>
-            <span className="text-muted text-xs">{(cfg.durationMs / 1000).toFixed(0)}s</span>
+            <span className="text-muted text-xs">
+              {formatUnit(Math.round(cfg.durationMs / 1000), "second", lang)}
+            </span>
           </div>
           <Slider
             className="mt-1"
@@ -522,7 +525,8 @@ function SignInMethods({ me }: { me: Me }) {
 // and does a one-time "Plex is master" import. Admin-only (rides the server admin token) and only
 // shown when the Plex module is enabled. Two-way push/pull arrive in later phases.
 function PlexWatchSync() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const link = useQuery({ queryKey: qk.plexWatchLink(), queryFn: () => api.plexWatchLink() });
   const enabled = link.data?.sync_enabled ?? false;
@@ -565,7 +569,9 @@ function PlexWatchSync() {
   // Reflect the toggle's target immediately (optimistic) so the switch responds on click rather than
   // only flipping once the multi-second import returns — otherwise it reads as "nothing happened".
   const shown = toggle.isPending ? Boolean(toggle.variables) : enabled;
-  const last = link.data?.last_watch_sync_at ? formatDateTime(link.data.last_watch_sync_at) : null;
+  const last = link.data?.last_watch_sync_at
+    ? formatDateTime(link.data.last_watch_sync_at, lang)
+    : null;
 
   return (
     <Section title={t("settings.plexSync.title")}>
@@ -765,7 +771,8 @@ export function DownloadAuth() {
 /** Who is signed in: name, address, role — and the latest sign-in on any device, so one the user does
  *  not recognise is something they can see. */
 export function AccountHeader({ me }: { me: Me }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   return (
     <Section title={t("settings.account.title")}>
       <div className="flex items-center gap-3 mb-3">
@@ -782,7 +789,7 @@ export function AccountHeader({ me }: { me: Me }) {
           {!me.is_demo && (
             <div className="text-xs text-muted" data-testid="account-last-login">
               {me.last_login_at
-                ? t("settings.account.lastLogin", { time: formatDateTime(me.last_login_at) })
+                ? t("settings.account.lastLogin", { time: formatDateTime(me.last_login_at, lang) })
                 : t("settings.account.lastLoginNone")}
             </div>
           )}

frontend/src/components/Sidebar.tsx

@@ -22,6 +22,7 @@ import { useFeedFilters, useFeedFiltersActions } from "./feedFiltersContext";
 import { useChannelsActions } from "./channelsContext";
 import { useMe } from "../lib/useMe";
 import { useTags } from "../lib/useTags";
+import { compareText } from "../lib/collate";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
 const DATE_PRESETS: { days: number; key: string }[] = [
@@ -47,7 +48,8 @@ export default function Sidebar({
   collapsed: boolean;
   onToggleCollapse: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const isDemo = useMe().is_demo;
   const { openModal } = useSavedViews();
   const filters = useFeedFilters();
@@ -76,7 +78,9 @@ export default function Sidebar({
     const shown = facetsReady
       ? list.filter((tg) => chipCount(tg) > 0 || filters.tags.includes(tg.id))
       : list;
-    return [...shown].sort((a, b) => chipCount(b) - chipCount(a) || a.name.localeCompare(b.name));
+    return [...shown].sort(
+      (a, b) => chipCount(b) - chipCount(a) || compareText(a.name, b.name, lang),
+    );
   };
 
   const languages = tags.filter((t) => t.category === "language");

frontend/src/components/Stats.tsx

@@ -66,7 +66,8 @@ export default function Stats() {
 
 // Per-user view: sync status + your own API usage + (for real accounts) the manual sync actions.
 function Overview({ me }: { me: Me }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   // The 403 case needs somewhere to go, or the toast names a fix the user cannot reach from here.
   const { openWizard } = useWizard();
@@ -125,14 +126,14 @@ function Overview({ me }: { me: Me }) {
                 label={t("stats.sync.fullHistoryEta")}
                 hint={t("stats.sync.fullHistoryEtaHint", { count: s.deep_pending_count })}
               >
-                {formatEta(s.deep_eta_seconds)}
+                {formatEta(t, s.deep_eta_seconds)}
               </SettingRow>
             )}
             <SettingRow label={t("stats.sync.myVideos")} hint={t("stats.sync.myVideosHint")}>
-              {formatNumber(s.my_videos)}
+              {formatNumber(s.my_videos, lang)}
             </SettingRow>
             <SettingRow label={t("stats.sync.quotaLeft")} hint={t("stats.sync.quotaLeftHint")}>
-              {formatNumber(s.quota_remaining_today)}
+              {formatNumber(s.quota_remaining_today, lang)}
             </SettingRow>
           </div>
         ) : status.isError ? (
@@ -149,16 +150,16 @@ function Overview({ me }: { me: Me }) {
           <>
             <div className="space-y-1.5 text-sm">
               <SettingRow label={t("stats.sync.today")} hint={t("stats.sync.todayHint")}>
-                {formatNumber(usage.data.today)}
+                {formatNumber(usage.data.today, lang)}
               </SettingRow>
               <SettingRow label={t("stats.sync.last7d")}>
-                {formatNumber(usage.data.last_7d)}
+                {formatNumber(usage.data.last_7d, lang)}
               </SettingRow>
               <SettingRow label={t("stats.sync.last30d")}>
-                {formatNumber(usage.data.last_30d)}
+                {formatNumber(usage.data.last_30d, lang)}
               </SettingRow>
               <SettingRow label={t("stats.sync.allTime")}>
-                {formatNumber(usage.data.all_time)}
+                {formatNumber(usage.data.all_time, lang)}
               </SettingRow>
             </div>
             {Object.keys(usage.data.by_action).length > 0 && (
@@ -168,8 +169,8 @@ function Overview({ me }: { me: Me }) {
                   .sort((a, b) => b[1] - a[1])
                   .map(([action, units]) => (
                     <div key={action} className="flex justify-between gap-2">
-                      <span>{quotaActionLabel(action)}</span>
-                      <span className="tabular-nums">{formatNumber(units)}</span>
+                      <span>{quotaActionLabel(t, action)}</span>
+                      <span className="tabular-nums">{formatNumber(units, lang)}</span>
                     </div>
                   ))}
               </div>
@@ -218,7 +219,8 @@ function Overview({ me }: { me: Me }) {
 
 // Admin-only view: instance-wide quota dashboard + the global background-sync pause toggle.
 function SystemStats() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [days, setDays] = useState<number>(30);
   const q = useQuery({ queryKey: qk.adminQuota(days), queryFn: () => api.adminQuota(days) });
@@ -263,7 +265,7 @@ function SystemStats() {
           className="flex items-center gap-1.5"
           options={RANGES.map((d) => ({
             value: String(d),
-            label: t("stats.rangeDays", { d: formatUnit(d, "day") }),
+            label: t("stats.rangeDays", { d: formatUnit(d, "day", lang) }),
           }))}
           value={String(days)}
           onChange={(v) => setDays(Number(v))}
@@ -290,8 +292,8 @@ function SystemStats() {
           <div className={`${SURFACE.card} p-3 mb-4`}>
             <div className="text-xs text-muted mb-2">
               {t("stats.dailyTotal", {
-                d: formatUnit(data.range_days, "day"),
-                units: formatNumber(grandTotal),
+                d: formatUnit(data.range_days, "day", lang),
+                units: formatNumber(grandTotal, lang),
               })}
             </div>
             {data.daily.length === 0 ? (
@@ -304,7 +306,10 @@ function SystemStats() {
                     // The day under the pointer is lit by the B3 layer over its whole column, not by
                     // dimming the bar: an opacity dim reads as "disabled".
                     className="flex-1 h-full flex items-end bg-card/50 rounded-t state-layer"
-                    title={t("stats.dailyTooltip", { day: d.day, units: formatNumber(d.total) })}
+                    title={t("stats.dailyTooltip", {
+                      day: d.day,
+                      units: formatNumber(d.total, lang),
+                    })}
                   >
                     <div
                       className="w-full bg-accent rounded-t"
@@ -333,7 +338,8 @@ function SystemStats() {
 }
 
 function UserRow({ row, max }: { row: AdminQuotaRow; max: number }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const [open, setOpen] = useState(false);
   const actions = Object.entries(row.by_action).sort((a, b) => b[1] - a[1]);
   const isSystem = row.user_id === null;
@@ -349,7 +355,7 @@ function UserRow({ row, max }: { row: AdminQuotaRow; max: number }) {
           {isSystem ? t("stats.system") : row.email}
         </span>
         <span className="text-sm font-semibold tabular-nums shrink-0">
-          {formatNumber(row.total)}
+          {formatNumber(row.total, lang)}
         </span>
       </Pressable>
       {/* Proportion bar */}
@@ -363,8 +369,8 @@ function UserRow({ row, max }: { row: AdminQuotaRow; max: number }) {
         <div className="mt-2 space-y-1 text-xs text-muted">
           {actions.map(([action, units]) => (
             <div key={action} className="flex items-center justify-between gap-2">
-              <span>{quotaActionLabel(action)}</span>
-              <span className="tabular-nums">{formatNumber(units)}</span>
+              <span>{quotaActionLabel(t, action)}</span>
+              <span className="tabular-nums">{formatNumber(units, lang)}</span>
             </div>
           ))}
         </div>

frontend/src/components/SyncStatus.tsx

@@ -5,7 +5,7 @@ import { useTranslation } from "react-i18next";
 import { CheckCircle2, Clock, Database, History, Loader2, Pause, Play } from "lucide-react";
 import { api, type MyStatus } from "../lib/api";
 import { notifyActionError } from "../lib/actionErrors";
-import { formatViews } from "../lib/format";
+import { formatCompact } from "../lib/format";
 import { PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
 import { Button } from "./ui/Button";
@@ -23,7 +23,8 @@ export default function SyncStatus({
   isAdmin: boolean;
   onGoToFullHistory: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [open, setOpen] = useState(false);
   // A read-out with a couple of controls rather than a list of choices, so it is a `dialog`
@@ -114,8 +115,9 @@ export default function SyncStatus({
   // popover. An accent dot flags the only two things worth interrupting you for (paused / missing
   // history) so the collapsed chip still tells you when to look.
   const needsAttention = data.paused || notFull > 0;
-  const countsText = `${formatViews(data.my_videos)} ${t("header.sync.yours")} / ${formatViews(
+  const countsText = `${formatCompact(data.my_videos, lang)} ${t("header.sync.yours")} / ${formatCompact(
     data.total_videos,
+    lang,
   )} ${t("header.sync.total")}`;
   return (
     <div className="pointer-events-auto shrink-0">
@@ -132,7 +134,9 @@ export default function SyncStatus({
         ) : (
           <Database className="w-3.5 h-3.5 shrink-0" />
         )}
-        <span className="text-fg font-medium tabular-nums">{formatViews(data.my_videos)}</span>
+        <span className="text-fg font-medium tabular-nums">
+          {formatCompact(data.my_videos, lang)}
+        </span>
         {needsAttention && (
           <span className="absolute -top-0.5 -right-0.5 w-2 h-2 rounded-full bg-accent ring-2 ring-bg" />
         )}
@@ -149,10 +153,10 @@ export default function SyncStatus({
         >
           <Database className="w-3.5 h-3.5 shrink-0" />
           <span>
-            <span className="text-fg font-medium">{formatViews(data.my_videos)}</span>{" "}
+            <span className="text-fg font-medium">{formatCompact(data.my_videos, lang)}</span>{" "}
             {t("header.sync.yours")}
             <span className="opacity-40"> / </span>
-            {formatViews(data.total_videos)} {t("header.sync.total")}
+            {formatCompact(data.total_videos, lang)} {t("header.sync.total")}
           </span>
         </div>
         {showMain && (

frontend/src/components/UpNextCard.dom.test.tsx

@@ -11,7 +11,10 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 vi.mock("../lib/api", () => ({

frontend/src/components/UpNextCard.tsx

@@ -23,8 +23,8 @@ export function UpNextCard({
   onAdvance: () => void;
   onCancel: () => void;
 }) {
-  const { t } = useTranslation();
-  const lang = plexLang();
+  const { t, i18n } = useTranslation();
+  const lang = plexLang(i18n.language);
   const nextQ = useQuery({
     queryKey: qk.plexItem(nextId, lang),
     queryFn: () => api.plexItem(nextId, lang),

frontend/src/components/UploadDialog.dom.test.tsx

@@ -19,7 +19,10 @@ const h = vi.hoisted(() => ({
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 vi.mock("../lib/api", () => ({
   api: { uploadDownload: h.uploadDownload, downloadUsage: h.downloadUsage },

frontend/src/components/UploadDialog.tsx

@@ -21,7 +21,8 @@ import { Input } from "./ui/Input";
 // file is streamed to the server via XHR so this dialog can show real upload progress. On success it
 // lands in the Library as an already-`done` job, so we invalidate + point the user there.
 export default function UploadDialog({ onClose }: { onClose: () => void }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const qc = useQueryClient();
   const [file, setFile] = useState<File | null>(null);
   const [name, setName] = useState("");
@@ -148,7 +149,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
           <>
             <FileIcon className="w-6 h-6 text-muted" />
             <span className="text-sm font-medium break-all">{file.name}</span>
-            <span className="text-xs text-muted">{formatBytes(file.size)}</span>
+            <span className="text-xs text-muted">{formatBytes(file.size, lang)}</span>
           </>
         ) : (
           <>
@@ -172,7 +173,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
 
       {tooBig && (
         <p key={pickSeq} id={refusalId} className="text-xs text-danger mt-2" role="alert">
-          {t("downloads.errors.upload.tooLarge", { size: formatBytes(cap ?? 0) })}
+          {t("downloads.errors.upload.tooLarge", { size: formatBytes(cap ?? 0, lang) })}
         </p>
       )}
 

frontend/src/components/VideoCard.tsx

@@ -17,7 +17,13 @@ import { navItemAttrs } from "../lib/listNav";
 import clsx from "clsx";
 import type { Video, VideoStatus } from "../lib/api";
 import { FEED_VIEW_SPEC, type FeedView } from "../lib/feedView";
-import { formatDate, formatDuration, formatNumber, formatViews, relativeTime } from "../lib/format";
+import {
+  formatDate,
+  formatDuration,
+  formatNumber,
+  formatCompact,
+  relativeTime,
+} from "../lib/format";
 import { isPlainLeftClick } from "../lib/nav";
 import { endsTextSelection } from "../lib/textSelection";
 import ChannelNameLink from "./ChannelNameLink";
@@ -391,6 +397,7 @@ function VideoCard({
   shelf?: boolean;
 }) {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const { openChannel } = useNavigationActions();
   const spec = FEED_VIEW_SPEC[view];
   const watched = video.status === "watched";
@@ -405,14 +412,14 @@ function VideoCard({
   // on every line and the tooltip carries the figure regardless.
   const viewsCell = (withWord: boolean) =>
     video.view_count != null && (
-      <span title={`${formatNumber(video.view_count)} ${t("card.views")}`}>
-        {formatViews(video.view_count)}
+      <span title={`${formatNumber(video.view_count, lang)} ${t("card.views")}`}>
+        {formatCompact(video.view_count, lang)}
         {withWord && ` ${t("card.views")}`}
       </span>
     );
   const published = (
     <>
-      {relativeTime(video.published_at)}
+      {relativeTime(video.published_at, lang)}
       {video.published_at && (
         <>
           {" · "}

frontend/src/components/VideoInfoCard.tsx

@@ -3,7 +3,7 @@ import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { api, type Video } from "../lib/api";
-import { formatDate, formatDuration, formatViews } from "../lib/format";
+import { formatDate, formatDuration, formatCompact } from "../lib/format";
 import { linkify } from "../lib/linkify";
 import Avatar from "./ui/Avatar";
 import { PopoverSurface } from "./ui/AnchoredPopover";
@@ -34,6 +34,7 @@ export default function VideoDetailsAt({
   onClose: () => void;
 }) {
   const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const pop = useAnchoredPopover({
     open: true,
     onOpenChange: (next) => {
@@ -72,7 +73,7 @@ export default function VideoDetailsAt({
 
   const description = detail?.description?.trim();
   const metaBits = [
-    video.view_count != null ? `${formatViews(video.view_count)} ${t("card.views")}` : null,
+    video.view_count != null ? `${formatCompact(video.view_count, lang)} ${t("card.views")}` : null,
     video.duration_seconds != null ? formatDuration(video.duration_seconds) : null,
     video.published_at ? formatDate(video.published_at, i18n.language) : null,
   ].filter(Boolean);

frontend/src/components/WatchPage.tsx

@@ -317,7 +317,7 @@ export default function WatchPage() {
                 {meta.title || T.file}
               </h1>
               <div className="text-sm text-muted mt-1">
-                {[meta.container?.toUpperCase(), formatBytes(meta.size_bytes)]
+                {[meta.container?.toUpperCase(), formatBytes(meta.size_bytes, lang)]
                   .filter(Boolean)
                   .join(" · ")}
               </div>

frontend/src/components/channelCells.tsx

@@ -12,7 +12,8 @@ import { isFullySynced } from "./channelSync";
 // full history are both in — folding the retired Sync column's at-a-glance status into the value it
 // describes. Full detail stays in the hover tooltip.
 export function StoredCell({ c }: { c: ManagedChannel }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   return (
     <span className="inline-flex items-center justify-end gap-1 text-muted tabular-nums">
       {isFullySynced(c) && (
@@ -24,7 +25,7 @@ export function StoredCell({ c }: { c: ManagedChannel }) {
           />
         </Tooltip>
       )}
-      {formatNumber(c.stored_videos)}
+      {formatNumber(c.stored_videos, lang)}
     </span>
   );
 }

frontend/src/components/channelColumns.dom.test.tsx

@@ -78,6 +78,7 @@ describe("buildChannelColumns — the row's end", () => {
     const noop = () => {};
     const cols = buildChannelColumns({
       t: ((k: string) => k) as never,
+      lang: "en",
       prioOptions: [],
       userTags: [],
       bumpPriority: noop,

frontend/src/components/channelColumns.tsx

@@ -14,7 +14,10 @@ import { RelativeTime } from "./ui/RelativeTime";
 /** The shared "subscribers" DataTable column (right-aligned, sortable, em-dash for null) — identical
  * in the Channels manager and the ChannelDiscovery table, which show subscriber counts on different
  * channel row shapes. */
-export function subsColumn<T extends { subscriber_count: number | null }>(t: TFunction): Column<T> {
+export function subsColumn<T extends { subscriber_count: number | null }>(
+  t: TFunction,
+  lang: string,
+): Column<T> {
   return {
     key: "subs",
     header: t("channels.cols.subs"),
@@ -23,7 +26,7 @@ export function subsColumn<T extends { subscriber_count: number | null }>(t: TFu
     sortable: true,
     sortValue: (c) => c.subscriber_count ?? -1,
     render: (c) => (
-      <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count)}</span>
+      <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count, lang)}</span>
     ),
   };
 }
@@ -50,6 +53,8 @@ export function tagsColumnTest(c: ManagedChannel, values: string[]): boolean {
  * column block lives outside the page component (its cell components are in channelCells). */
 export interface ChannelColumnDeps {
   t: TFunction;
+  /** The UI language, for the numbers — the compiler re-builds the columns when it changes. */
+  lang: string;
   prioOptions: { value: string; label: string }[];
   userTags: ComponentProps<typeof ChannelTagsControl>["userTags"];
   bumpPriority: (id: string, delta: number) => void;
@@ -69,6 +74,7 @@ export interface ChannelColumnDeps {
 export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChannel>[] {
   const {
     t,
+    lang,
     prioOptions,
     userTags,
     bumpPriority,
@@ -144,7 +150,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       // The stored count carries the "fully synced" ✓ charm (R14 folded the retired Sync column in).
       render: (c) => <StoredCell c={c} />,
     },
-    { ...subsColumn<ManagedChannel>(t), width: "6rem" },
+    { ...subsColumn<ManagedChannel>(t, lang), width: "6rem" },
     {
       key: "lastUpload",
       header: t("channels.cols.lastUpload"),
@@ -165,7 +171,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       sortValue: (c) => c.total_duration_seconds,
       render: (c) => (
         <span className="text-muted tabular-nums">
-          {formatTotalHours(c.total_duration_seconds)}
+          {formatTotalHours(c.total_duration_seconds, lang)}
         </span>
       ),
     },

frontend/src/components/comfortPin.langSwitch.dom.test.tsx

@@ -0,0 +1,34 @@
+// @vitest-environment jsdom
+//
+// E30 S3: a Plex query follows a language switch. `plexLang()` read the i18n singleton, and under the
+// React Compiler a value with no reactive input is computed once: after switching to Hungarian the
+// comfort shelf kept asking for its English cards. The language now comes off useTranslation. Real
+// i18n (the key-echo mock does not re-render on a switch, which is the very thing under test).
+import { describe, it, expect, afterEach, vi } from "vitest";
+import type { ReactNode } from "react";
+import { renderHook, act, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { setLanguage } from "../i18n";
+import { api } from "../lib/api";
+import { useComfortPin } from "./comfortPin";
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  setLanguage("en");
+});
+
+function wrapper({ children }: { children: ReactNode }) {
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return <QueryClientProvider client={qc}>{children}</QueryClientProvider>;
+}
+
+describe("the comfort shelf after a language switch", () => {
+  it("asks for its cards in the new language", async () => {
+    const get = vi.spyOn(api, "comfortPins").mockImplementation(() => new Promise(() => {}));
+    setLanguage("en");
+    renderHook(() => useComfortPin(true), { wrapper });
+    await waitFor(() => expect(get).toHaveBeenCalledWith("en"));
+    act(() => setLanguage("hu"));
+    await waitFor(() => expect(get).toHaveBeenCalledWith("hu"));
+  });
+});

frontend/src/components/comfortPin.tsx

@@ -95,7 +95,7 @@ export interface ComfortPin {
  * every feed load would be a real request for a string almost nobody reads that session.
  */
 export function useComfortPin(open: boolean): ComfortPin {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   // Latch: react-query keeps the data cached when `enabled` goes false again, so this only decides
   // WHEN the first fetch happens, not whether the answer survives the menu closing.
@@ -105,8 +105,8 @@ export function useComfortPin(open: boolean): ComfortPin {
   if (open && !everOpen) setEverOpen(true);
 
   const shelf = useQuery({
-    queryKey: qk.comfortPins(plexLang()),
-    queryFn: () => api.comfortPins(plexLang()),
+    queryKey: qk.comfortPins(plexLang(i18n.language)),
+    queryFn: () => api.comfortPins(plexLang(i18n.language)),
     enabled: everOpen,
     // The shelf is hand-curated and short; re-asking on every menu open would be noise.
     staleTime: 60_000,

frontend/src/components/downloadAdmin.tsx

@@ -160,7 +160,8 @@ function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }
 }
 
 export function AdminSystem() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const storage = useLiveQuery(qk.adminStorage(), api.adminDownloadStorage, { intervalMs: 5000 });
   const jobs = useLiveQuery(qk.adminDownloads(), api.adminDownloads, { intervalMs: 5000 });
   const [quotaFor, setQuotaFor] = useState<{ id: number; email: string } | null>(null);
@@ -171,10 +172,10 @@ export function AdminSystem() {
         <div className="grid grid-cols-2 sm:grid-cols-4 gap-3 mb-5">
           {[
             [t("downloads.admin.readyFiles"), String(s.ready_files)],
-            [t("downloads.admin.totalSize"), formatBytes(s.total_bytes)],
+            [t("downloads.admin.totalSize"), formatBytes(s.total_bytes, lang)],
             [
               t("downloads.admin.cap"),
-              s.total_cap_bytes ? formatBytes(s.total_cap_bytes) : t("downloads.admin.noCap"),
+              s.total_cap_bytes ? formatBytes(s.total_cap_bytes, lang) : t("downloads.admin.noCap"),
             ],
           ].map(([label, val]) => (
             <div key={label} className={`${SURFACE.card} p-3`}>
@@ -199,7 +200,7 @@ export function AdminSystem() {
                 className="flex items-center gap-2 text-sm px-3 py-1.5 rounded-lg bg-card/40"
               >
                 <span className="flex-1 truncate">{u.email}</span>
-                <span className="text-muted">{formatBytes(u.footprint_bytes)}</span>
+                <span className="text-muted">{formatBytes(u.footprint_bytes, lang)}</span>
                 <IconButton
                   onClick={() => setQuotaFor({ id: u.user_id, email: u.email ?? "" })}
                   label={t("downloads.admin.editQuota")}
@@ -226,7 +227,7 @@ export function AdminSystem() {
             <span className="text-muted truncate w-40">{j.user_email}</span>
             <StatusPill job={j} />
             <span className="text-muted w-16 text-right">
-              {j.asset?.size_bytes ? formatBytes(j.asset.size_bytes) : ""}
+              {j.asset?.size_bytes ? formatBytes(j.asset.size_bytes, lang) : ""}
             </span>
           </div>
         ))}

frontend/src/components/downloadRow.dom.test.tsx

@@ -9,7 +9,10 @@ import { render, screen, cleanup, fireEvent } from "@testing-library/react";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 import { DownloadRow, IconBtn } from "./downloadRow";

frontend/src/components/downloadRow.tsx

@@ -240,7 +240,8 @@ export function DownloadRow({
   more?: ReactNode;
   children?: ReactNode;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const running = job.status === "running";
   const linkTitle = !hasOwnTitle(job) && !!job.source_url;
   const sourceLink = linkTitle ? null : job.source_url;
@@ -294,16 +295,16 @@ export function DownloadRow({
             <span className="text-muted uppercase"> · {job.asset.container}</span>
           ) : null}
           {job.asset?.size_bytes ? (
-            <span className="text-muted"> · {formatBytes(job.asset.size_bytes)}</span>
+            <span className="text-muted"> · {formatBytes(job.asset.size_bytes, lang)}</span>
           ) : null}
           {job.asset?.duration_s ? (
             <span className="text-muted"> · {formatDuration(job.asset.duration_s)}</span>
           ) : null}
           {job.status === "done" && job.created_at ? (
-            <span className="text-muted" title={formatDateTime(job.created_at)}>
+            <span className="text-muted" title={formatDateTime(job.created_at, lang)}>
               {" · "}
               <CalendarClock className="inline w-3.5 h-3.5 align-[-0.15em]" />{" "}
-              {relativeTime(job.created_at)}
+              {relativeTime(job.created_at, lang)}
             </span>
           ) : null}
           {job.status === "error" ? (
@@ -360,8 +361,8 @@ export function DownloadRow({
                 </div>
                 <div className="text-xs text-muted mt-0.5 flex gap-2">
                   <span>{job.progress}%</span>
-                  {job.speed_bps ? <span>{formatSpeed(job.speed_bps)}</span> : null}
-                  {job.eta_s ? <span>{formatEta(job.eta_s)}</span> : null}
+                  {job.speed_bps ? <span>{formatSpeed(job.speed_bps, lang)}</span> : null}
+                  {job.eta_s ? <span>{formatEta(t, job.eta_s)}</span> : null}
                 </div>
               </>
             )}
@@ -373,7 +374,8 @@ export function DownloadRow({
 }
 
 export function UsageBar() {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // Poll live (like the library list) so the footprint updates the moment the worker finishes an
   // edit/download — enqueue only creates a queued job (0 bytes), the size lands asynchronously, so
   // a one-shot fetch at enqueue time would show a stale 0 B until a manual reload. The footprint
@@ -391,8 +393,8 @@ export function UsageBar() {
       <div className="flex justify-between text-sm mb-1.5">
         <span className="font-medium">{t("downloads.usage.title")}</span>
         <span className="text-muted">
-          {formatBytes(u.footprint_bytes)}
-          {u.unlimited ? "" : ` / ${formatBytes(u.max_bytes)}`} ·{" "}
+          {formatBytes(u.footprint_bytes, lang)}
+          {u.unlimited ? "" : ` / ${formatBytes(u.max_bytes, lang)}`} ·{" "}
           {u.unlimited
             ? t("downloads.usage.unlimited")
             : t("downloads.usage.items", { used: u.active_jobs, max: u.max_jobs })}

frontend/src/components/plexBrowseLists.tsx

@@ -19,10 +19,10 @@ import { SURFACE } from "./ui/surfaces";
 
 // A collection tile's sub-line: item count · total runtime (each dropped when unknown, e.g. show-only
 // collections have no movie runtime).
-function collectionMeta(c: PlexCollection, t: TFunction): string {
+function collectionMeta(c: PlexCollection, t: TFunction, lang: string): string {
   const parts: string[] = [];
   if (c.child_count != null) parts.push(t("plex.browse.items", { count: c.child_count }));
-  const rt = formatRuntime(c.total_runtime_s);
+  const rt = formatRuntime(c.total_runtime_s, lang);
   if (rt) parts.push(rt);
   return parts.join(" · ");
 }
@@ -162,7 +162,7 @@ function NewCollectionPanel({
 
 // A grid of Plex collections (searchable — the union can run to hundreds); a tile opens its info page.
 export function PlexCollectionsBrowse({ onPick }: { onPick: (id: string, title: string) => void }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   const isAdmin = qc.getQueryData<{ role?: string }>(qk.me())?.role === "admin";
   const [search, setSearch] = useState("");
@@ -223,7 +223,7 @@ export function PlexCollectionsBrowse({ onPick }: { onPick: (id: string, title:
               navIndex={i}
               title={c.title}
               art={c.art}
-              subtitle={collectionMeta(c, t)}
+              subtitle={collectionMeta(c, t, i18n.language)}
               topRight={
                 isIntrinsicReadOnly(c) ? (
                   <Lock

frontend/src/components/plexCards.dom.test.tsx

@@ -9,7 +9,10 @@ import { render, screen, cleanup, fireEvent, act } from "@testing-library/react"
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 import { PlexPosterCard, SeasonCard, EpisodeCard } from "./plexCards";

frontend/src/components/plexCards.lang.dom.test.tsx

@@ -11,7 +11,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
   const t = (key: string, o?: Record<string, unknown>) =>
     o ? `${key}(${Object.values(o).join("|")})` : key;
-  return { ...actual, useTranslation: () => ({ t }) };
+  return { ...actual, useTranslation: () => ({ i18n: { language: "en" }, t }) };
 });
 
 import { PlexPosterCard, SeasonCard } from "./plexCards";

frontend/src/components/plexCards.langSwitch.dom.test.tsx

@@ -0,0 +1,35 @@
+// @vitest-environment jsdom
+//
+// E30 S3: a card's numbers follow a language switch. The formatters read the UI language, but the
+// React Compiler memoizes a call on its ARGUMENTS — `formatRuntime(card.duration_seconds)` stayed
+// "2 hr 4 min" after switching to Hungarian while the hero beside it said "2 ó 16 p". Real catalog.
+import { describe, it, expect, afterEach, vi } from "vitest";
+import { render, screen, cleanup, act } from "@testing-library/react";
+import { setLanguage } from "../i18n";
+import { PlexPosterCard } from "./plexCards";
+import type { PlexCard } from "../lib/api";
+
+afterEach(() => {
+  cleanup();
+  setLanguage("en");
+});
+
+const movie: PlexCard = {
+  id: "m1",
+  type: "movie",
+  title: "Heat",
+  thumb: "/api/plex/image/m1",
+  duration_seconds: 2 * 3600 + 4 * 60,
+  rating: 8.3,
+};
+
+describe("a card after a language switch", () => {
+  it("says its runtime and rating in the new language", () => {
+    setLanguage("en");
+    render(<PlexPosterCard card={movie} onOpen={vi.fn()} />);
+    expect(screen.getByText("2 hr 4 min")).toBeTruthy();
+    act(() => setLanguage("hu"));
+    expect(screen.getByText("2 ó 4 p")).toBeTruthy();
+    expect(screen.getByText("8,3")).toBeTruthy();
+  });
+});

frontend/src/components/plexCards.tsx

@@ -2,7 +2,7 @@ import { useTranslation } from "react-i18next";
 import { CheckCircle2, Circle, Info, ListPlus, Star } from "lucide-react";
 import clsx from "clsx";
 import { type ReactNode, useRef } from "react";
-import { formatRuntime } from "../lib/format";
+import { formatDecimal, formatRuntime } from "../lib/format";
 import { Carousel } from "./ui/Carousel";
 import type { PlexCard, PlexCastMember, PlexFilters, PlexSeasonDetail } from "../lib/api";
 import { seasonTitle } from "../lib/plexDetail";
@@ -287,7 +287,8 @@ export function PlexPosterCard({
   /** The show's page, beside a `next` episode. */
   onInfo?: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const preview = useMediaPreview();
   // An episode in a poster rail (Discover's "Recently watched") stands as its SHOW: the show's poster
   // and name, the episode on the line under it (E30, option A). Its own 16:9 still in a poster slot
@@ -302,7 +303,7 @@ export function PlexPosterCard({
   const acted = ep ?? card;
   const watched = card.status === "watched";
   const actedWatched = acted.status === "watched";
-  const runtime = formatRuntime(card.duration_seconds);
+  const runtime = formatRuntime(card.duration_seconds, lang);
   const pct = ep ? progressPct(ep) : watched ? 0 : progressPct(card);
   const m = more(card, menuButtonProps, menuOpen);
   const toggle = isPlayable && onToggleWatched ? () => onToggleWatched(acted) : undefined;
@@ -317,7 +318,7 @@ export function PlexPosterCard({
   const rating = card.rating != null && (
     <span className="inline-flex items-center gap-0.5">
       <Star className="w-3 h-3 fill-current text-rating" aria-hidden />
-      {card.rating.toFixed(1)}
+      {formatDecimal(card.rating, lang)}
     </span>
   );
   return (
@@ -649,11 +650,12 @@ export function EpisodeCard({
   menuOpen?: boolean;
   navIndex?: number;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const watched = ep.status === "watched";
   const pct = watched ? 0 : progressPct(ep);
   const m = more(ep, menuButtonProps, menuOpen);
-  const runtime = formatRuntime(ep.duration_seconds);
+  const runtime = formatRuntime(ep.duration_seconds, lang);
   return (
     <MediaCard
       dense

frontend/src/components/plexPlayerControls.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { RotateCcw } from "lucide-react";
 import { IconButton } from "./ui/IconButton";
 import { Slider as Range, CheckRow } from "./ui/NativeControls";
+import { formatDecimal, formatUnit } from "../lib/format";
 
 // --- settings-panel building blocks ------------------------------------------------------------
 export function PanelHead({ children }: { children: ReactNode }) {
@@ -17,6 +18,7 @@ export function Slider({
   max,
   step,
   suffix,
+  seconds,
   onChange,
   onReset,
 }: {
@@ -25,20 +27,24 @@ export function Slider({
   min: number;
   max: number;
   step: number;
+  /** A symbol after the number ("%", "px"). */
   suffix?: string;
+  /** The value is seconds: named in the UI language's unit ("0.5 sec" / "0,5 mp"). */
+  seconds?: boolean;
   onChange: (v: number) => void;
   onReset?: () => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
+  const shown = seconds
+    ? formatUnit(value, "second", lang)
+    : `${formatDecimal(value, lang, 1, 0)}${suffix ?? ""}`;
   return (
     <div className="px-1 py-1">
       <div className="mb-1 flex items-center justify-between text-[0.75rem] text-white/80">
         <span>{label}</span>
         <span className="flex items-center gap-1.5">
-          <span className="tabular-nums text-white/60">
-            {Number.isInteger(value) ? value : value.toFixed(1)}
-            {suffix ?? ""}
-          </span>
+          <span className="tabular-nums text-white/60">{shown}</span>
           {onReset && (
             <IconButton
               variant="overlay"

frontend/src/components/plexViews.restart.dom.test.tsx

@@ -21,7 +21,10 @@ beforeAll(() => {
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 const ep = (id: string, season: number, episode: number, position = 0) => ({

frontend/src/components/plexViews.tsx

@@ -58,6 +58,7 @@ import { Pressable } from "./ui/Pressable";
 import { SURFACE } from "./ui/surfaces";
 import { PageHeading } from "./ui/Headings";
 import { vocab } from "../lib/vocab";
+import { formatDate, formatDecimal } from "../lib/format";
 
 // The Plex sub-router's view branches, split out of PlexBrowse. Each is invoked by PlexBrowse with
 // shell callbacks as props (onBack/onPlay/onFilter/…) — no view reaches back into the shell's state,
@@ -82,8 +83,8 @@ export function PlexInfoView({
   onFilter: (patch: Partial<PlexFilters>) => void;
   onOpenPerson: (name: string) => void;
 }) {
-  const { t } = useTranslation();
-  const lang = plexLang();
+  const { t, i18n } = useTranslation();
+  const lang = plexLang(i18n.language);
   const q = useQuery({ queryKey: qk.plexItem(id, lang), queryFn: () => api.plexItem(id, lang) });
   return (
     <div className="w-[90%] max-w-400 mx-auto">
@@ -137,13 +138,13 @@ export function PlexPersonView({
   onInfoCard: (card: PlexCard) => void;
   onFilter: (patch: Partial<PlexFilters>) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   // ONE menu for this whole page — the "known for" rail and the two filmography grids are three
   // lists on one screen, and the popover anchors where it opened, so a second controller would only
   // mean a second closed popover to keep shut (see ui/ContextMenu).
   const menu = useContextMenu<PlexCard>();
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const personKey = qk.plexPerson(name, lang);
   const q = useQuery({ queryKey: personKey, queryFn: () => api.plexPerson(name, lang) });
   // E5 S8: cross-dissolve the backdrop through the person's films' art (the touch the roadmap calls
@@ -255,12 +256,13 @@ export function PlexPersonView({
                   {(d.birthday || d.place_of_birth) && (
                     <p>
                       <span className="text-fg/70">{t("plex.person.born")}:</span>{" "}
-                      {[d.birthday, d.place_of_birth].filter(Boolean).join(" · ")}
+                      {[formatDate(d.birthday, lang), d.place_of_birth].filter(Boolean).join(" · ")}
                     </p>
                   )}
                   {d.deathday && (
                     <p>
-                      <span className="text-fg/70">{t("plex.person.died")}:</span> {d.deathday}
+                      <span className="text-fg/70">{t("plex.person.died")}:</span>{" "}
+                      {formatDate(d.deathday, lang)}
                     </p>
                   )}
                   {d.also_known_as.length > 0 && (
@@ -345,10 +347,10 @@ export function PlexShowView({
   onFilter?: (patch: Partial<PlexFilters>) => void;
   onOpenPerson?: (name: string) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   const isAdmin = qc.getQueryData<{ role?: string }>(qk.me())?.role === "admin";
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const q = useQuery({
     queryKey: qk.plexShow(showId, lang),
     queryFn: () => api.plexShow(showId, lang),
@@ -535,7 +537,7 @@ export function PlexShowView({
                   >
                     {/* the rating gold — its own token, not --warn: a score is not a warning */}
                     <Star className="w-3.5 h-3.5 text-rating" fill="currentColor" />
-                    {score.toFixed(1)}
+                    {formatDecimal(score, lang)}
                   </Pressable>
                 );
               })()}
@@ -671,9 +673,9 @@ export function PlexSeasonView({
   onBack: () => void;
   onPlay: (epRk: string, queue: string[], fromStart?: boolean) => void;
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const q = useQuery({
     queryKey: qk.plexShow(showId, lang),
     queryFn: () => api.plexShow(showId, lang),

frontend/src/components/ui/DataTable.fixedLayout.dom.test.tsx

@@ -10,7 +10,10 @@ import DataTable, { type Column } from "./DataTable";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 afterEach(cleanup);

frontend/src/components/ui/DataTable.keyboard.dom.test.tsx

@@ -10,7 +10,10 @@ import DataTable, { type Column } from "./DataTable";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 
 afterEach(() => {

frontend/src/components/ui/DataTable.tsx

@@ -132,7 +132,8 @@ export default function DataTable<T>({
   // the hidden-filter cue leaves those out, it would name them twice.
   surfacedFilters?: string[];
 }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const initial = loadPersist(persistKey);
   // Sort is controlled when the caller passes onSortChange (it then owns/persists it); otherwise
   // the table keeps its own, seeded from and saved to persistKey.
@@ -212,7 +213,7 @@ export default function DataTable<T>({
     setPage(0);
   }, [resetFiltersToken, commitFilters]);
 
-  const sorted = sortRows(filterRows(rows, columns, filters), columns, sort);
+  const sorted = sortRows(filterRows(rows, columns, filters), columns, sort, lang);
 
   // size <= 0 means "All" — one page with every row.
   const allRows = size <= 0;

frontend/src/components/ui/HiddenFilterCue.dom.test.tsx

@@ -13,6 +13,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   return {
     ...actual,
     useTranslation: () => ({
+      i18n: { language: "en" },
       t: (key: string, o?: { name?: string; count?: number }) =>
         o?.name ? `${key}(${o.name})` : o?.count !== undefined ? `${key}#${o.count}` : key,
     }),

frontend/src/components/ui/ItemCount.dom.test.tsx

@@ -30,7 +30,7 @@ describe("ItemCount", () => {
     render(<ItemCount count={192045} />);
     // Separators come from the shared formatNumber — compare against it, not a fixed en-US string,
     // so the test asserts ItemCount's formatting path rather than the CI locale.
-    expect(text()).toBe(`${formatNumber(192045)} items`);
+    expect(text()).toBe(`${formatNumber(192045, "en")} items`);
   });
 
   // The pill used `count.toLocaleString()`, which groups in the BROWSER's locale: a Hungarian UI on
@@ -52,7 +52,9 @@ describe("ItemCount", () => {
   it("collapses a count of a million or more to 1M+, with the exact figure in the tooltip", () => {
     render(<ItemCount count={1_000_000} />);
     expect(text()).toBe("1M+ items"); // the "1M+" collapse is locale-independent
-    expect(screen.getByTestId("item-count").getAttribute("title")).toBe(formatNumber(1_000_000));
+    expect(screen.getByTestId("item-count").getAttribute("title")).toBe(
+      formatNumber(1_000_000, "en"),
+    );
   });
 
   it("shows the icon-only fixed box (empty label) while the count is undefined", () => {

frontend/src/components/ui/ItemCount.tsx

@@ -14,7 +14,8 @@ const OVERFLOW_AT = 1_000_000;
 // "1M+", with the exact figure in the title tooltip. Deliberately generic ("item(s)", dynamic plural)
 // so any surface can reuse it for whatever it counts.
 export default function ItemCount({ count }: { count?: number }) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   // `count` is undefined only on a first, un-cached load — keep the fixed box (nothing shifts), just
   // show the icon until the number arrives.
   const label =
@@ -22,12 +23,12 @@ export default function ItemCount({ count }: { count?: number }) {
       ? ""
       : t("common.itemCount", {
           count,
-          formattedCount: count >= OVERFLOW_AT ? "1M+" : formatNumber(count),
+          formattedCount: count >= OVERFLOW_AT ? "1M+" : formatNumber(count, lang),
         });
   return (
     <span
       data-testid="item-count"
-      title={count == null ? undefined : formatNumber(count)}
+      title={count == null ? undefined : formatNumber(count, lang)}
       className="inline-flex h-7 min-w-[9rem] shrink-0 items-center justify-center gap-1.5 rounded-full bg-fg/8 px-3 text-sm whitespace-nowrap text-muted"
     >
       <Layers className="h-3.5 w-3.5 shrink-0 text-muted" aria-hidden="true" />

frontend/src/components/ui/RelativeTime.dom.test.tsx

@@ -4,6 +4,7 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { cleanup, render } from "@testing-library/react";
 import { formatDateTime, formatDateTimeSec, relativeTime } from "../../lib/format";
+import "../../i18n"; // the real instance, in English: the component names time in the UI language
 import { RelativeTime } from "./RelativeTime";
 
 afterEach(cleanup);
@@ -14,8 +15,8 @@ describe("RelativeTime", () => {
   it("says how long ago, and names the exact minute on hover", () => {
     const { container } = render(<RelativeTime at={THREE_DAYS_AGO} />);
     const el = container.querySelector("time")!;
-    expect(el.textContent).toBe(relativeTime(THREE_DAYS_AGO));
-    expect(el.getAttribute("title")).toBe(formatDateTime(THREE_DAYS_AGO));
+    expect(el.textContent).toBe(relativeTime(THREE_DAYS_AGO, "en"));
+    expect(el.getAttribute("title")).toBe(formatDateTime(THREE_DAYS_AGO, "en"));
     expect(el.getAttribute("dateTime")).toBe(THREE_DAYS_AGO);
   });
 
@@ -23,7 +24,7 @@ describe("RelativeTime", () => {
     const ms = Date.parse(THREE_DAYS_AGO);
     const { container } = render(<RelativeTime at={ms} seconds />);
     expect(container.querySelector("time")!.getAttribute("title")).toBe(
-      formatDateTimeSec(THREE_DAYS_AGO),
+      formatDateTimeSec(THREE_DAYS_AGO, "en"),
     );
   });
 

frontend/src/components/ui/RelativeTime.tsx

@@ -1,3 +1,4 @@
+import { useTranslation } from "react-i18next";
 import { formatDateTime, formatDateTimeSec, relativeFromMs } from "../../lib/format";
 
 // docs/DESIGN.md "Copy": a relative time ("3d ago") says how recent something is in a list, and the
@@ -16,14 +17,15 @@ export function RelativeTime({
   empty?: string;
   className?: string;
 }) {
+  const lang = useTranslation().i18n.language;
   const ms = typeof at === "number" ? at : at ? new Date(at).getTime() : NaN;
-  const text = Number.isFinite(ms) ? relativeFromMs(ms) : "";
+  const text = Number.isFinite(ms) ? relativeFromMs(ms, lang) : "";
   if (!text) return empty ? <span className={className}>{empty}</span> : null;
   const iso = new Date(ms).toISOString();
   return (
     <time
       dateTime={iso}
-      title={seconds ? formatDateTimeSec(iso) : formatDateTime(iso)}
+      title={seconds ? formatDateTimeSec(iso, lang) : formatDateTime(iso, lang)}
       className={className}
     >
       {text}

frontend/src/components/useChannelActions.dom.test.tsx

@@ -18,7 +18,10 @@ const h = vi.hoisted(() => ({
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
-  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+  return {
+    ...actual,
+    useTranslation: () => ({ i18n: { language: "en" }, t: (key: string) => key }),
+  };
 });
 vi.mock("../lib/notifications", async (importOriginal) => ({
   ...(await importOriginal<typeof import("../lib/notifications")>()),

frontend/src/components/useChannelActions.tsx

@@ -5,6 +5,7 @@ import { qk } from "../lib/queryKeys";
 import { invalidateSubscription } from "../lib/subscriptionCache";
 import { api, type ManagedChannel, type Tag } from "../lib/api";
 import { priorityChoices } from "../lib/channelOrder";
+import { compareText } from "../lib/collate";
 import { channelYouTubeUrl } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -40,7 +41,8 @@ export function useChannelActions({
   onPickPriority: (c: ManagedChannel, priority: number) => void;
 }) {
   const qc = useQueryClient();
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
+  const lang = i18n.language;
   const confirm = useConfirm();
   const me = useMe();
   const isAdmin = me.role === "admin";
@@ -166,7 +168,7 @@ export function useChannelActions({
     return {
       ...base,
       items: [...tags]
-        .sort((a, b) => a.name.localeCompare(b.name))
+        .sort((a, b) => compareText(a.name, b.name, lang))
         .map((tg) => ({
           id: `tag-${tg.id}`,
           label: tg.name,

frontend/src/components/usePlexPlaylistEdits.ts

@@ -13,12 +13,12 @@ import { reinsertAt } from "../lib/reinsert";
  *  Split from `PlexPlaylistView` so the failure paths are reachable from a test: the reorder is
  *  driven by a dnd-kit drag, and neither of its sensors activates under jsdom. */
 export function usePlexPlaylistEdits(playlistId: number) {
-  const { t } = useTranslation();
+  const { t, i18n } = useTranslation();
   const qc = useQueryClient();
   const [busy, setBusy] = useState(false);
   // The rows' titles are in the reader's language (E30), so the cache entry is per language — and ONE
   // key serves the read and every write below (setQueryData matches exactly).
-  const lang = plexLang();
+  const lang = plexLang(i18n.language);
   const key = qk.plexPlaylist(playlistId, lang);
   const q = useQuery({
     queryKey: key,

frontend/src/i18n/index.ts

@@ -67,11 +67,13 @@ export function setLanguage(code: LangCode): void {
   document.documentElement.lang = code;
 }
 
-// The Plex overlay serves only EN or HU metadata (E5 S6). Collapse the active UI language to one of
-// those for the `?lang=` param / query keys. Components read `i18n.language` off useTranslation (which
-// re-renders on switch); this singleton reader is for non-component code (e.g. plexPlayerHooks).
-export function plexLang(): "en" | "hu" {
-  return i18n.language?.startsWith("hu") ? "hu" : "en";
+// The Plex overlay serves only EN or HU metadata (E5 S6). Collapse a UI language to one of those for
+// the `?lang=` param / query keys. It takes the language instead of reading the singleton (E30 S3):
+// under the React Compiler a component's `plexLang()` was a value with no reactive input, so it kept
+// the language the component first rendered in — a switch left every Plex query on the old language.
+// A component passes `i18n.language` from useTranslation; non-component code the singleton's.
+export function plexLang(lang: string | undefined): "en" | "hu" {
+  return lang?.startsWith("hu") ? "hu" : "en";
 }
 
 export default i18n;

frontend/src/lib/api/core.ts

@@ -4,7 +4,7 @@
 // so the runtime graph is a clean barrel → domain → core with no cycles.
 import { getActiveToasts, notify, remove } from "../notifications";
 import i18n from "../../i18n";
-import { formatBytes } from "../format";
+import { formatBytes, formatDecimal } from "../format";
 import { reportError } from "../errorDialog";
 import { activeAccountId, pinActiveAccount } from "../activeAccount";
 import { logRequest, QUIET_HEADER, REQUEST_ID_HEADER, ROUTE_HEADER, send } from "../http";
@@ -86,9 +86,7 @@ function localizeDetail(d: unknown): string {
     | undefined;
   if (o?.code === "quota") {
     if (o.reason === "max_bytes") {
-      const gb = new Intl.NumberFormat(i18n.language, { maximumFractionDigits: 1 }).format(
-        (o.limit || 0) / 1_073_741_824,
-      );
+      const gb = formatDecimal((o.limit || 0) / 1_073_741_824, i18n.language, 1, 0);
       return t("downloads.errors.quota.max_bytes", { size: gb });
     }
     if (o.reason === "max_jobs") return t("downloads.errors.quota.max_jobs", { limit: o.limit });
@@ -117,7 +115,7 @@ function localizeDetail(d: unknown): string {
       : t("welcome.auth.deadLink.short");
   // Local-upload ingest (E7 S5): the per-file cap carries the limit in bytes; empty is a bare code.
   if (o?.code === "upload_too_large")
-    return t("downloads.errors.upload.tooLarge", { size: formatBytes(o.limit) });
+    return t("downloads.errors.upload.tooLarge", { size: formatBytes(o.limit, i18n.language) });
   if (o?.code === "upload_empty") return t("downloads.errors.upload.empty");
   // The EDGE ceiling's refusal (`bodylimit.TOO_LARGE_BODY`), shared by every route and carrying no
   // number. Not a theoretical case: on a DEFAULT install it is what an over-cap upload gets,

frontend/src/lib/collate.ts

@@ -0,0 +1,13 @@
+// Alphabetical order in the UI language (E30 S3): Hungarian puts "Ö" after "O" and "Cs" after "Cu". A
+// bare `localeCompare` compares in the BROWSER's locale; the server sorts by the same ICU rules
+// (backend `app/collation.py`), so a list sorted here agrees with one the server sorted. Its own
+// module, apart from lib/format.ts, because it needs no i18n: the pure sort helpers import it.
+const collators = new Map<string, Intl.Collator>();
+export function compareText(a: string, b: string, lang: string): number {
+  let c = collators.get(lang);
+  if (!c) {
+    c = new Intl.Collator(lang);
+    collators.set(lang, c);
+  }
+  return c.compare(a, b);
+}

frontend/src/lib/columnSort.test.ts

@@ -27,37 +27,51 @@ const columns = [
 
 describe("sortRows", () => {
   it("returns the same array reference when there is no sort", () => {
-    expect(sortRows(rows, columns, null)).toBe(rows);
+    expect(sortRows(rows, columns, null, "en")).toBe(rows);
   });
 
   it("returns the input unchanged for a column that has no sortValue", () => {
-    expect(sortRows(rows, columns, { key: "nosort", dir: "asc" })).toBe(rows);
+    expect(sortRows(rows, columns, { key: "nosort", dir: "asc" }, "en")).toBe(rows);
   });
 
   it("sorts numbers by numeric difference, not string order", () => {
     // string order would put "100" before "30"; numeric must not.
-    const asc = sortRows(rows, columns, { key: "views", dir: "asc" }).map((r) => r.views);
+    const asc = sortRows(rows, columns, { key: "views", dir: "asc" }, "en").map((r) => r.views);
     expect(asc).toEqual([5, 30, 100]);
-    const desc = sortRows(rows, columns, { key: "views", dir: "desc" }).map((r) => r.views);
+    const desc = sortRows(rows, columns, { key: "views", dir: "desc" }, "en").map((r) => r.views);
     expect(desc).toEqual([100, 30, 5]);
   });
 
   it("sorts strings with locale compare, ascending and descending", () => {
-    expect(sortRows(rows, columns, { key: "name", dir: "asc" }).map((r) => r.name)).toEqual([
+    expect(sortRows(rows, columns, { key: "name", dir: "asc" }, "en").map((r) => r.name)).toEqual([
       "alpha",
       "beta",
       "gamma",
     ]);
-    expect(sortRows(rows, columns, { key: "name", dir: "desc" }).map((r) => r.name)).toEqual([
+    expect(sortRows(rows, columns, { key: "name", dir: "desc" }, "en").map((r) => r.name)).toEqual([
       "gamma",
       "beta",
       "alpha",
     ]);
   });
 
+  // E30 S3: text sorts in the UI language's alphabet, not the browser's — Hungarian puts "Ö" after
+  // "O" and "Cs" after "Cu". Both languages are asserted and differ, so a bare `localeCompare` (one
+  // runtime locale for both) cannot pass on any machine.
+  it("sorts text in the UI language's alphabet", () => {
+    const words = ["Zorró", "Ötödik", "oroszlán", "Csillag", "Életrajzi", "Cukor"].map((name) => ({
+      name,
+      views: 0,
+    }));
+    const by = (lang: string) =>
+      sortRows(words, columns, { key: "name", dir: "asc" }, lang).map((r) => r.name);
+    expect(by("hu")).toEqual(["Cukor", "Csillag", "Életrajzi", "oroszlán", "Ötödik", "Zorró"]);
+    expect(by("en")).toEqual(["Csillag", "Cukor", "Életrajzi", "oroszlán", "Ötödik", "Zorró"]);
+  });
+
   it("does not mutate the input array", () => {
     const before = rows.map((r) => r.name);
-    sortRows(rows, columns, { key: "name", dir: "asc" });
+    sortRows(rows, columns, { key: "name", dir: "asc" }, "en");
     expect(rows.map((r) => r.name)).toEqual(before);
   });
 });

frontend/src/lib/columnSort.ts

@@ -2,6 +2,8 @@
 // manager's card layout) can sort the same rows the same way — one comparator, one source of truth.
 // Kept structural (no `Column` import) so it has zero coupling to DataTable.
 
+import { compareText } from "./collate";
+
 export type SortState = { key: string; dir: "asc" | "desc" } | null;
 
 interface Sortable<T> {
@@ -9,9 +11,10 @@ interface Sortable<T> {
   sortValue?: (row: T) => string | number;
 }
 
-/** Sort a copy of `rows` by the sortable column named in `sort` (numeric diff or locale compare).
- * Returns `rows` unchanged when there's no sort or the column has no `sortValue`. */
-export function sortRows<T>(rows: T[], columns: Sortable<T>[], sort: SortState): T[] {
+/** Sort a copy of `rows` by the sortable column named in `sort` (numeric diff, or text in the UI
+ * language's alphabet — `lang`). Returns `rows` unchanged when there's no sort or the column has no
+ * `sortValue`. */
+export function sortRows<T>(rows: T[], columns: Sortable<T>[], sort: SortState, lang: string): T[] {
   if (!sort) return rows;
   const get = columns.find((c) => c.key === sort.key)?.sortValue;
   if (!get) return rows;
@@ -21,7 +24,7 @@ export function sortRows<T>(rows: T[], columns: Sortable<T>[], sort: SortState):
     const cmp =
       typeof av === "number" && typeof bv === "number"
         ? av - bv
-        : String(av).localeCompare(String(bv));
+        : compareText(String(av), String(bv), lang);
     return sort.dir === "asc" ? cmp : -cmp;
   });
 }

frontend/src/lib/feedSort.ts

@@ -3,6 +3,7 @@
 // chips can label a sort without re-deriving the mapping.
 import type { Video } from "./api";
 import type { SortOption } from "./columnSort";
+import { compareText } from "./collate";
 
 export type SortKey =
   | "date"
@@ -100,7 +101,12 @@ const YT_SORT_VALUE: Partial<Record<SortKey, (v: Video) => number | string | nul
  *  Results missing the value sink to the END in both directions (a video whose duration YouTube did
  *  not report is not "the shortest"), and ties keep the provider's ranking, in both directions —
  *  hence a signed comparator rather than sorting and reversing, which would flip the ties too. */
-export function sortYtResults(items: Video[], key: SortKey, dir: "asc" | "desc"): Video[] {
+export function sortYtResults(
+  items: Video[],
+  key: SortKey,
+  dir: "asc" | "desc",
+  lang: string,
+): Video[] {
   const value = YT_SORT_VALUE[key];
   if (!value) return items; // relevance, and anything a stale pref names that we no longer offer
   const sign = dir === "asc" ? 1 : -1;
@@ -109,7 +115,7 @@ export function sortYtResults(items: Video[], key: SortKey, dir: "asc" | "desc")
   have.sort((a, b) => {
     const x = value(a)!;
     const y = value(b)!;
-    return sign * (typeof x === "string" ? x.localeCompare(String(y)) : x - Number(y));
+    return sign * (typeof x === "string" ? compareText(x, String(y), lang) : x - Number(y));
   });
   return [...have, ...missing];
 }

frontend/src/lib/feedSort.yt.test.ts

@@ -46,7 +46,7 @@ describe("YT_SORT_KEYS", () => {
 
 describe("sortYtResults", () => {
   it("leaves the provider's ranking alone on `relevance`", () => {
-    expect(titles(sortYtResults(RESULTS, "relevance", "desc"))).toEqual([
+    expect(titles(sortYtResults(RESULTS, "relevance", "desc", "en"))).toEqual([
       "b-old-short",
       "a-new-long",
       "c-mid-mid",
@@ -54,12 +54,12 @@ describe("sortYtResults", () => {
   });
 
   it("orders by upload date in both directions", () => {
-    expect(titles(sortYtResults(RESULTS, "date", "desc"))).toEqual([
+    expect(titles(sortYtResults(RESULTS, "date", "desc", "en"))).toEqual([
       "a-new-long",
       "c-mid-mid",
       "b-old-short",
     ]);
-    expect(titles(sortYtResults(RESULTS, "date", "asc"))).toEqual([
+    expect(titles(sortYtResults(RESULTS, "date", "asc", "en"))).toEqual([
       "b-old-short",
       "c-mid-mid",
       "a-new-long",
@@ -67,14 +67,18 @@ describe("sortYtResults", () => {
   });
 
   it("orders by views and by length", () => {
-    expect(titles(sortYtResults(RESULTS, "popular", "desc"))[0]).toBe("c-mid-mid");
-    expect(titles(sortYtResults(RESULTS, "duration", "desc"))[0]).toBe("a-new-long");
-    expect(titles(sortYtResults(RESULTS, "duration", "asc"))[0]).toBe("b-old-short");
+    expect(titles(sortYtResults(RESULTS, "popular", "desc", "en"))[0]).toBe("c-mid-mid");
+    expect(titles(sortYtResults(RESULTS, "duration", "desc", "en"))[0]).toBe("a-new-long");
+    expect(titles(sortYtResults(RESULTS, "duration", "asc", "en"))[0]).toBe("b-old-short");
   });
 
   it("orders by title alphabetically, not by code point", () => {
     const accented = [v({ title: "Zebra" }), v({ title: "Ábel" }), v({ title: "alma" })];
-    expect(titles(sortYtResults(accented, "title", "asc"))).toEqual(["Ábel", "alma", "Zebra"]);
+    expect(titles(sortYtResults(accented, "title", "asc", "en"))).toEqual([
+      "Ábel",
+      "alma",
+      "Zebra",
+    ]);
   });
 
   it("sinks results with no value to the END in both directions", () => {
@@ -84,8 +88,16 @@ describe("sortYtResults", () => {
       v({ title: "long", duration_seconds: 900 }),
       v({ title: "short", duration_seconds: 60 }),
     ];
-    expect(titles(sortYtResults(mixed, "duration", "desc"))).toEqual(["long", "short", "unknown"]);
-    expect(titles(sortYtResults(mixed, "duration", "asc"))).toEqual(["short", "long", "unknown"]);
+    expect(titles(sortYtResults(mixed, "duration", "desc", "en"))).toEqual([
+      "long",
+      "short",
+      "unknown",
+    ]);
+    expect(titles(sortYtResults(mixed, "duration", "asc", "en"))).toEqual([
+      "short",
+      "long",
+      "unknown",
+    ]);
   });
 
   it("keeps the provider's ranking inside a tie, in both directions", () => {
@@ -95,19 +107,27 @@ describe("sortYtResults", () => {
       v({ title: "second", view_count: 100 }),
       v({ title: "third", view_count: 5 }),
     ];
-    expect(titles(sortYtResults(tied, "popular", "desc"))).toEqual(["first", "second", "third"]);
-    expect(titles(sortYtResults(tied, "popular", "asc"))).toEqual(["third", "first", "second"]);
+    expect(titles(sortYtResults(tied, "popular", "desc", "en"))).toEqual([
+      "first",
+      "second",
+      "third",
+    ]);
+    expect(titles(sortYtResults(tied, "popular", "asc", "en"))).toEqual([
+      "third",
+      "first",
+      "second",
+    ]);
   });
 
   it("does not reorder anything for a key it cannot honour", () => {
     // A pref stored before this view offered a control can name `shuffle` or `priority`.
-    expect(titles(sortYtResults(RESULTS, "priority", "desc"))).toEqual(titles(RESULTS));
-    expect(titles(sortYtResults(RESULTS, "shuffle", "desc"))).toEqual(titles(RESULTS));
+    expect(titles(sortYtResults(RESULTS, "priority", "desc", "en"))).toEqual(titles(RESULTS));
+    expect(titles(sortYtResults(RESULTS, "shuffle", "desc", "en"))).toEqual(titles(RESULTS));
   });
 
   it("does not mutate the array it was given", () => {
     const input = [...RESULTS];
-    sortYtResults(input, "date", "asc");
+    sortYtResults(input, "date", "asc", "en");
     expect(titles(input)).toEqual(titles(RESULTS));
   });
 });

frontend/src/lib/format.test.ts

@@ -1,6 +1,11 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
   RELATIVE_UNITS,
+  formatBytes,
+  formatCompact,
+  formatCountOrDash,
+  formatDecimal,
+  formatMoney,
   formatDate,
   formatDateTime,
   formatDateTimeSec,
@@ -69,29 +74,29 @@ describe("relativeFromMs", () => {
 
   for (const [secs, tier, expectedEn, expectedHu] of CASES) {
     it(`${secs}s is ${tier} — "${expectedEn}" / "${expectedHu}"`, () => {
-      expect(relativeFromMs(NOW - secs * 1000)).toBe(expectedEn);
+      expect(relativeFromMs(NOW - secs * 1000, i18nStub.language)).toBe(expectedEn);
       i18nStub.language = "hu";
-      expect(relativeFromMs(NOW - secs * 1000)).toBe(expectedHu);
+      expect(relativeFromMs(NOW - secs * 1000, i18nStub.language)).toBe(expectedHu);
     });
   }
 
   it("follows a language switch at runtime", () => {
     const twoHoursAgo = NOW - 2 * HOUR * 1000;
-    expect(relativeFromMs(twoHoursAgo)).toBe("2h ago");
+    expect(relativeFromMs(twoHoursAgo, i18nStub.language)).toBe("2h ago");
     i18nStub.language = "hu";
-    expect(relativeFromMs(twoHoursAgo)).toBe("2 órája");
+    expect(relativeFromMs(twoHoursAgo, i18nStub.language)).toBe("2 órája");
     i18nStub.language = "en";
-    expect(relativeFromMs(twoHoursAgo)).toBe("2h ago");
+    expect(relativeFromMs(twoHoursAgo, i18nStub.language)).toBe("2h ago");
   });
 
   it("clamps a future timestamp instead of counting up", () => {
-    expect(relativeFromMs(NOW + 5 * DAY * 1000)).toBe("time.justNow");
+    expect(relativeFromMs(NOW + 5 * DAY * 1000, i18nStub.language)).toBe("time.justNow");
   });
 
   it("renders nothing for a timestamp it cannot read", () => {
-    expect(relativeTime("not-a-date")).toBe("");
-    expect(relativeTime(null)).toBe("");
-    expect(relativeFromMs(NaN)).toBe("");
+    expect(relativeTime("not-a-date", i18nStub.language)).toBe("");
+    expect(relativeTime(null, i18nStub.language)).toBe("");
+    expect(relativeFromMs(NaN, i18nStub.language)).toBe("");
   });
 
   // Derived from RELATIVE_UNITS, not from CASES, so adding a tier to the table without covering
@@ -135,9 +140,9 @@ describe("formatNumber", () => {
   });
 
   it("groups in the UI language, not in the runtime's", () => {
-    const asEnglish = formatNumber(1234567);
+    const asEnglish = formatNumber(1234567, i18nStub.language);
     i18nStub.language = "hu";
-    const asHungarian = formatNumber(1234567);
+    const asHungarian = formatNumber(1234567, i18nStub.language);
     expect(asEnglish).toBe(new Intl.NumberFormat("en").format(1234567));
     expect(asHungarian).toBe(new Intl.NumberFormat("hu").format(1234567));
     // The load-bearing half: under `toLocaleString()` both come out in the runtime's one locale.
@@ -145,24 +150,26 @@ describe("formatNumber", () => {
   });
 
   it("follows a language switch at runtime (the per-language cache is keyed, not frozen)", () => {
-    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("en").format(1000));
+    expect(formatNumber(1000, i18nStub.language)).toBe(new Intl.NumberFormat("en").format(1000));
     i18nStub.language = "hu";
-    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("hu").format(1000));
+    expect(formatNumber(1000, i18nStub.language)).toBe(new Intl.NumberFormat("hu").format(1000));
     i18nStub.language = "en";
-    expect(formatNumber(1000)).toBe(new Intl.NumberFormat("en").format(1000));
+    expect(formatNumber(1000, i18nStub.language)).toBe(new Intl.NumberFormat("en").format(1000));
   });
 
   it("renders nothing for a value it cannot read, so a caller's dash fallback covers it", () => {
-    expect(formatNumber(null)).toBe("");
-    expect(formatNumber(undefined)).toBe("");
-    expect(formatNumber(NaN)).toBe("");
-    expect(formatNumber(Infinity)).toBe("");
-    expect(formatNumber(0)).toBe("0"); // a real zero is a value, not a missing one
+    expect(formatNumber(null, i18nStub.language)).toBe("");
+    expect(formatNumber(undefined, i18nStub.language)).toBe("");
+    expect(formatNumber(NaN, i18nStub.language)).toBe("");
+    expect(formatNumber(Infinity, i18nStub.language)).toBe("");
+    expect(formatNumber(0, i18nStub.language)).toBe("0"); // a real zero is a value, not a missing one
   });
 
   it("carries the UI language into the channel total-hours label", () => {
     i18nStub.language = "hu";
-    expect(formatTotalHours(12345 * 3600)).toBe(`${new Intl.NumberFormat("hu").format(12345)} ó`);
+    expect(formatTotalHours(12345 * 3600, i18nStub.language)).toBe(
+      `${new Intl.NumberFormat("hu").format(12345)} ó`,
+    );
   });
 });
 
@@ -185,18 +192,18 @@ describe("formatRuntime", () => {
   ];
 
   it.each(CASES)("formats %o as %o", (sec, expected) => {
-    expect(formatRuntime(sec)).toBe(expected);
+    expect(formatRuntime(sec, i18nStub.language)).toBe(expected);
   });
 
   it("speaks the UI language's units (DESIGN Copy: Intl short), not English letters", () => {
     i18nStub.language = "hu";
-    expect(formatRuntime(5025)).toBe("1 ó 23 p");
+    expect(formatRuntime(5025, i18nStub.language)).toBe("1 ó 23 p");
     i18nStub.language = "en";
   });
 
   it("never reports a non-zero duration as zero", () => {
     for (let sec = 1; sec < 3600; sec++) {
-      expect(formatRuntime(sec), `${sec}s`).not.toBe("0 min");
+      expect(formatRuntime(sec, i18nStub.language), `${sec}s`).not.toBe("0 min");
     }
   });
 });
@@ -216,9 +223,9 @@ describe("formatDateTime / formatDate", () => {
 
   it("formats a timestamp in the UI language, not in the runtime's", () => {
     const iso = "2026-09-17T08:30:14Z";
-    const asEnglish = formatDateTime(iso);
+    const asEnglish = formatDateTime(iso, i18nStub.language);
     i18nStub.language = "hu";
-    const asHungarian = formatDateTime(iso);
+    const asHungarian = formatDateTime(iso, i18nStub.language);
     expect(asEnglish).not.toBe(asHungarian);
     expect(formatDate(iso, "en")).not.toBe(formatDate(iso, "hu"));
   });
@@ -226,10 +233,10 @@ describe("formatDateTime / formatDate", () => {
   it("names the month and stops at the minute; seconds only in the log form (DESIGN Copy)", () => {
     process.env.TZ = "UTC";
     const iso = "2026-09-17T08:30:14Z";
-    expect(formatDateTime(iso)).toBe("Sep 17, 2026, 08:30 AM");
-    expect(formatDateTimeSec(iso)).toBe("Sep 17, 2026, 08:30:14 AM");
+    expect(formatDateTime(iso, i18nStub.language)).toBe("Sep 17, 2026, 08:30 AM");
+    expect(formatDateTimeSec(iso, i18nStub.language)).toBe("Sep 17, 2026, 08:30:14 AM");
     i18nStub.language = "hu";
-    expect(formatDateTime(iso)).toBe("2026. szept. 17. 08:30");
+    expect(formatDateTime(iso, i18nStub.language)).toBe("2026. szept. 17. 08:30");
     expect(formatDate(iso, "hu")).toBe("2026. szept. 17.");
   });
 
@@ -240,3 +247,39 @@ describe("formatDateTime / formatDate", () => {
     expect(formatDate("2026-09-17", "en")).toBe("Sep 17, 2026");
   });
 });
+
+// E30 S3: every number a reader sees is in the UI language — the decimal mark, the grouping and the
+// compact suffix. `toFixed` and hand-built "1.2K" wrote English in both languages. Both languages are
+// asserted and differ, so an implementation that ignores the language cannot pass.
+describe("numbers in the UI language", () => {
+  it("writes a rating with the language's decimal mark", () => {
+    expect(formatDecimal(8.25, "en")).toBe("8.3");
+    expect(formatDecimal(8.25, "hu")).toBe("8,3");
+    expect(formatDecimal(8, "hu")).toBe("8,0"); // a rating keeps its decimal
+    expect(formatDecimal(5, "hu", 1, 0)).toBe("5"); // unless asked to drop a zero one
+  });
+
+  it("writes a big count compact, with the language's suffix", () => {
+    // Intl keeps the number and its suffix together with a no-break space
+    const hu = (n: number) => formatCompact(n, "hu").replace(/\u00a0/g, " ");
+    expect(formatCompact(1234, "en")).toBe("1.2K");
+    expect(hu(1234)).toBe("1,2 E");
+    expect(formatCompact(3_400_000, "en")).toBe("3.4M");
+    expect(hu(3_400_000)).toBe("3,4 M");
+    expect(formatCompact(999, "hu")).toBe("999");
+    expect(formatCompact(null, "hu")).toBe("");
+    expect(formatCountOrDash(null, "hu")).toBe("—");
+  });
+
+  it("writes a file size with the language's decimal mark", () => {
+    expect(formatBytes(5 * 1024 ** 3, "en")).toBe("5.0 GB");
+    expect(formatBytes(5 * 1024 ** 3, "hu")).toBe("5,0 GB");
+    expect(formatBytes(501747, "hu")).toBe("490 KB");
+    expect(formatBytes(0, "hu")).toBe("0 B");
+  });
+
+  it("writes a dollar sum compact, never padded", () => {
+    expect(formatMoney(160_000_000, "en")).toBe("$160M");
+    expect(formatMoney(160_000_000, "hu")).not.toBe(formatMoney(160_000_000, "en"));
+  });
+});

frontend/src/lib/format.ts

@@ -1,12 +1,36 @@
+import type { TFunction } from "i18next";
 import i18n from "../i18n";
 
-export function relativeTime(iso: string | null): string {
-  if (!iso) return "";
-  return relativeFromMs(new Date(iso).getTime());
+// The one home of the app's LOCALE forms — numbers, units, dates (alphabetical order: `collate.ts`) —
+// all in the UI language, never the browser's (lint: `local/locale-format` refuses a hand-built one elsewhere).
+//
+// Every formatter takes the language (or `t`) as an ARGUMENT, and that is load-bearing, not style
+// (E30 S3): the React Compiler memoizes a call on its arguments, so a formatter that read the language
+// off the i18n singleton kept its old answer after a language switch — a card still said "2 hr 4 min"
+// beside a hero saying "2 ó 16 p". Passing `i18n.language` (from useTranslation) makes the language a
+// dependency the compiler can see; the signature makes forgetting it a type error.
+
+// Intl formatters are expensive to build and every grid formats one per card: one per language ×
+// options, built lazily. Keyed by language because the user can switch it at runtime.
+const numberFormats = new Map<string, Intl.NumberFormat>();
+function numberFormat(lang: string, opts: Intl.NumberFormatOptions = {}): Intl.NumberFormat {
+  const key = `${lang}|${JSON.stringify(opts)}`;
+  let f = numberFormats.get(key);
+  if (!f) {
+    f = new Intl.NumberFormat(lang, opts);
+    numberFormats.set(key, f);
+  }
+  return f;
 }
 
-/** Human file size, binary units (1024). e.g. 501747 -> "490 KB", 5368709120 -> "5.0 GB". */
-export function formatBytes(bytes: number | null | undefined): string {
+export function relativeTime(iso: string | null, lang: string): string {
+  if (!iso) return "";
+  return relativeFromMs(new Date(iso).getTime(), lang);
+}
+
+/** Human file size, binary units (1024), the number in the UI language: 501747 -> "490 KB",
+ * 5368709120 -> "5.0 GB" / "5,0 GB". The unit symbols are the same in both languages. */
+export function formatBytes(bytes: number | null | undefined, lang: string): string {
   const n = Number(bytes);
   if (!n || n < 0) return "0 B";
   const units = ["B", "KB", "MB", "GB", "TB"];
@@ -16,13 +40,14 @@ export function formatBytes(bytes: number | null | undefined): string {
     v /= 1024;
     i++;
   }
-  return `${v >= 100 || i === 0 ? Math.round(v) : v.toFixed(1)} ${units[i]}`;
+  const num = v >= 100 || i === 0 ? formatNumber(Math.round(v), lang) : formatDecimal(v, lang);
+  return `${num} ${units[i]}`;
 }
 
 /** Download speed from bytes/sec, e.g. "2.4 MB/s". */
-export function formatSpeed(bps: number | null | undefined): string {
+export function formatSpeed(bps: number | null | undefined, lang: string): string {
   if (!bps) return "";
-  return `${formatBytes(bps)}/s`;
+  return `${formatBytes(bps, lang)}/s`;
 }
 
 /** Relative-time tiers as [seconds-per-unit, Intl unit]. The unit is what the divisor produces,
@@ -65,18 +90,18 @@ function relativeFormatter(lang: string): Intl.RelativeTimeFormat {
  * client-side notifications whose timestamps are already numbers). One implementation shared
  * instead of re-implemented per component. Empty string for a timestamp that can't be read, so a
  * caller's `|| "—"` fallback covers it like a missing one. */
-export function relativeFromMs(ms: number): string {
+export function relativeFromMs(ms: number, lang: string): string {
   const secs = Math.max(0, (Date.now() - ms) / 1000);
   if (!Number.isFinite(secs)) return "";
   // Sub-minute is the one case Intl has no wording for — it would say "0m ago".
-  if (secs < RELATIVE_UNITS[0][0]) return i18n.t("time.justNow");
+  if (secs < RELATIVE_UNITS[0][0]) return i18n.t("time.justNow", { lng: lang });
   let [div, unit] = RELATIVE_UNITS[0];
   for (const [unitSecs, unitName] of RELATIVE_UNITS) {
     if (secs < unitSecs) break;
     div = unitSecs;
     unit = unitName;
   }
-  return relativeFormatter(i18n.language).format(-Math.floor(secs / div), unit);
+  return relativeFormatter(lang).format(-Math.floor(secs / div), unit);
 }
 
 // The canonical YouTube URL for a channel: its @handle when known (nicer), else the
@@ -100,15 +125,17 @@ export function formatDuration(sec: number | null): string {
 // S3g): "90 min", "2 hr" / "90 p", "2 ó". Hand-typed "90m" / "2h" were English in both languages and
 // came in three spellings. One formatter per language × unit, built lazily (the grids format one per card).
 export type DurationUnit = "second" | "minute" | "hour" | "day";
-const unitFormatters = new Map<string, Intl.NumberFormat>();
-export function formatUnit(value: number, unit: DurationUnit): string {
-  const key = `${i18n.language}|${unit}`;
-  let f = unitFormatters.get(key);
-  if (!f) {
-    f = new Intl.NumberFormat(i18n.language, { style: "unit", unit, unitDisplay: "short" });
-    unitFormatters.set(key, f);
-  }
-  return f.format(value);
+export function formatUnit(
+  value: number,
+  unit: DurationUnit,
+  lang: string,
+  digits?: number,
+): string {
+  const fraction =
+    digits == null ? {} : { minimumFractionDigits: digits, maximumFractionDigits: digits };
+  return numberFormat(lang, { style: "unit", unit, unitDisplay: "short", ...fraction }).format(
+    value,
+  );
 }
 
 /** Compact human runtime ("1 hr 30 min", "45 min", "45 sec"; HU "1 ó 30 p") from seconds. Formats
@@ -118,70 +145,84 @@ export function formatUnit(value: number, unit: DurationUnit): string {
  * The sub-minute tier is load-bearing, not decoration: an aggregate is the only input that can land
  * between 1 and 59 seconds (a playlist of Shorts), and flooring that to a flat "0 min" made a
  * non-empty playlist report no runtime at all. Keep it, or that case silently regresses. */
-export function formatRuntime(sec?: number | null): string {
+export function formatRuntime(sec: number | null | undefined, lang: string): string {
   if (!sec) return "";
-  if (sec < 60) return formatUnit(Math.floor(sec), "second");
+  if (sec < 60) return formatUnit(Math.floor(sec), "second", lang);
   const h = Math.floor(sec / 3600);
   const m = Math.floor((sec % 3600) / 60);
-  return h ? `${formatUnit(h, "hour")} ${formatUnit(m, "minute")}` : formatUnit(m, "minute");
+  return h
+    ? `${formatUnit(h, "hour", lang)} ${formatUnit(m, "minute", lang)}`
+    : formatUnit(m, "minute", lang);
 }
 
 // Human label for a canonical quota-action key (see backend app.quota.QuotaAction). Resolved
 // from i18n (quotaActions.<key>) so it's translated in all UI languages; falls back to the raw
 // key for any unmapped/legacy value.
-export function quotaActionLabel(action: string): string {
-  return i18n.t(`quotaActions.${action}`, { defaultValue: action });
+export function quotaActionLabel(t: TFunction, action: string): string {
+  return t(`quotaActions.${action}`, { defaultValue: action });
 }
 
 /** Coarse, human ETA for a remaining duration in seconds ("~3 hours", "~2 days"). Translated
  * via the `time.eta.*` keys (was hardcoded English, violating the trilingual rule). */
-export function formatEta(seconds: number): string {
-  if (seconds <= 0) return i18n.t("time.eta.done");
+export function formatEta(t: TFunction, seconds: number): string {
+  if (seconds <= 0) return t("time.eta.done");
   const hours = seconds / 3600;
-  if (hours < 1) return i18n.t("time.eta.lessThanHour");
-  if (hours < 24) return i18n.t("time.eta.hours", { count: Math.round(hours) });
-  return i18n.t("time.eta.days", { count: Math.round(hours / 24) });
+  if (hours < 1) return t("time.eta.lessThanHour");
+  if (hours < 24) return t("time.eta.hours", { count: Math.round(hours) });
+  return t("time.eta.days", { count: Math.round(hours / 24) });
 }
 
 /** Compact whole-channel total length ("12 hr", "45 min"), hours-scale so H:MM:SS isn't huge. */
-export function formatTotalHours(sec: number): string {
+export function formatTotalHours(sec: number, lang: string): string {
   if (!sec) return "—";
   const h = Math.round(sec / 3600);
-  return h >= 1 ? formatUnit(h, "hour") : formatUnit(Math.max(1, Math.round(sec / 60)), "minute");
+  return h >= 1
+    ? formatUnit(h, "hour", lang)
+    : formatUnit(Math.max(1, Math.round(sec / 60)), "minute", lang);
 }
 
-// One grouped-number formatter per language, built lazily — the same reason as the relative-time
-// formatters above: the stats tables and the feed format a number per row, and constructing an Intl
-// formatter is the expensive part. Keyed by language because the user can switch it at runtime.
-const numberFormatters = new Map<string, Intl.NumberFormat>();
-
 /** A grouped number in the UI language ("1,234,567" in English, "1 234 567" in Hungarian). A bare
  * `n.toLocaleString()` groups in the BROWSER's locale instead, so an English UI in a Hungarian
  * browser showed space-grouped figures and vice versa — the number half of the same defect
  * `formatDateTime` fixes for dates. Empty string for a missing/non-finite value, so a caller's
  * `|| "—"` fallback covers it like a missing one. */
-export function formatNumber(n: number | null | undefined): string {
+export function formatNumber(n: number | null | undefined, lang: string): string {
   if (n == null || !Number.isFinite(n)) return "";
-  const lang = i18n.language;
-  let formatter = numberFormatters.get(lang);
-  if (!formatter) {
-    formatter = new Intl.NumberFormat(lang);
-    numberFormatters.set(lang, formatter);
-  }
-  return formatter.format(n);
+  return numberFormat(lang).format(n);
 }
 
-export function formatViews(n: number | null): string {
-  if (n == null) return "";
-  if (n >= 1e9) return `${(n / 1e9).toFixed(1).replace(/\.0$/, "")}B`;
-  if (n >= 1e6) return `${(n / 1e6).toFixed(1).replace(/\.0$/, "")}M`;
-  if (n >= 1e3) return `${(n / 1e3).toFixed(1).replace(/\.0$/, "")}K`;
-  return String(n);
+/** A number with `digits` decimals in the UI language — a rating "8.3" / "8,3" (a bare `toFixed`
+ * always wrote the English decimal point). `minDigits` below `digits` drops trailing zeros: "5" GB. */
+export function formatDecimal(n: number, lang: string, digits = 1, minDigits = digits): string {
+  const opts = { minimumFractionDigits: minDigits, maximumFractionDigits: digits };
+  return numberFormat(lang, opts).format(n);
+}
+
+/** A big count, compact, in the UI language: "1.2K" / "1,2 E", "3.4M" / "3,4 M". The minimum is
+ * pinned for the reason `formatMoney` gives. Empty for null. */
+export function formatCompact(n: number | null | undefined, lang: string): string {
+  if (n == null || !Number.isFinite(n)) return "";
+  const opts = { notation: "compact", minimumFractionDigits: 0, maximumFractionDigits: 1 } as const;
+  return numberFormat(lang, opts).format(n);
+}
+
+/** A US-dollar sum, compact ("$160M"): TMDB budgets and revenues are USD whatever the reader's
+ * language. The minimum is load-bearing, not a default written out: given only a maximum, a compact
+ * format is free to pad to it. The same options render "$160.0M" on Node 22 and "$160M" on Node 24
+ * (measured 2026-09-20), and browsers split along the same line. Pin it rather than inherit it. */
+export function formatMoney(n: number, lang: string): string {
+  return numberFormat(lang, {
+    notation: "compact",
+    style: "currency",
+    currency: "USD",
+    minimumFractionDigits: 0,
+    maximumFractionDigits: 1,
+  }).format(n);
 }
 
 // A count for a right-aligned table cell: the humanized number, or an em-dash when unknown (null).
-export function formatCountOrDash(n: number | null | undefined): string {
-  return n != null ? formatViews(n) : "—";
+export function formatCountOrDash(n: number | null | undefined, lang: string): string {
+  return n != null ? formatCompact(n, lang) : "—";
 }
 
 // The app's date forms (docs/DESIGN.md "Copy", E28 S3g), all in the UI language — never a bare
@@ -195,12 +236,12 @@ export function formatCountOrDash(n: number | null | undefined): string {
 const DAY: Intl.DateTimeFormatOptions = { year: "numeric", month: "short", day: "numeric" };
 const MINUTE: Intl.DateTimeFormatOptions = { ...DAY, hour: "2-digit", minute: "2-digit" };
 
-export function formatDateTime(iso: string | null | undefined): string {
-  return iso ? new Date(iso).toLocaleString(i18n.language, MINUTE) : "";
+export function formatDateTime(iso: string | null | undefined, lang: string): string {
+  return iso ? new Date(iso).toLocaleString(lang, MINUTE) : "";
 }
 
-export function formatDateTimeSec(iso: string | null | undefined): string {
-  return iso ? new Date(iso).toLocaleString(i18n.language, { ...MINUTE, second: "2-digit" }) : "";
+export function formatDateTimeSec(iso: string | null | undefined, lang: string): string {
+  return iso ? new Date(iso).toLocaleString(lang, { ...MINUTE, second: "2-digit" }) : "";
 }
 
 // Absolute day-precision date (e.g. "12 Jan 2017"), shown alongside a relative time. Empty for null.

frontend/src/lib/jobResult.dom.test.ts

@@ -12,12 +12,16 @@ describe("a job's result", () => {
   it("names each count, and a phase's own counts inside it", () => {
     setLanguage("hu");
     const t = i18n.getFixedT("hu");
-    expect(jobResultText(t, { new: 3, failed: 0 })).toBe("új videó: 3 · sikertelen: 0");
+    expect(jobResultText(t, { new: 3, failed: 0 }, "hu")).toBe("új videó: 3 · sikertelen: 0");
     expect(
-      jobResultText(t, {
-        recent: { channels_processed: 0, videos_added: 0, reason: "no service user" },
-        deep: { channels_processed: 2, videos_added: 5, reconciled_full: 1 },
-      }),
+      jobResultText(
+        t,
+        {
+          recent: { channels_processed: 0, videos_added: 0, reason: "no service user" },
+          deep: { channels_processed: 2, videos_added: 5, reconciled_full: 1 },
+        },
+        "hu",
+      ),
     ).toBe(
       "friss (feldolgozott csatorna: 0 · hozzáadott videó: 0 · ok: nincs szolgáltatásfiók) · " +
         "mély (feldolgozott csatorna: 2 · hozzáadott videó: 5 · teljesen egyeztetve: 1)",
@@ -26,15 +30,15 @@ describe("a job's result", () => {
 
   it("names a skip reason; an upstream error stays its own text", () => {
     const t = i18n.getFixedT("hu");
-    expect(jobResultText(t, { enriched: 0, skipped: "tmdb_api_key not set" })).toBe(
+    expect(jobResultText(t, { enriched: 0, skipped: "tmdb_api_key not set" }, "hu")).toBe(
       "kiegészítve: 0 · kihagyva: nincs TMDB API-kulcs",
     );
-    expect(jobResultText(t, { error: "Plex returned 500" })).toBe("hiba: Plex returned 500");
+    expect(jobResultText(t, { error: "Plex returned 500" }, "hu")).toBe("hiba: Plex returned 500");
   });
 
   it("shows a key or word it does not know as it is, readable", () => {
     const t = i18n.getFixedT("hu");
-    expect(jobResultText(t, { brand_new_count: 3, skipped: "moon is full" })).toBe(
+    expect(jobResultText(t, { brand_new_count: 3, skipped: "moon is full" }, "hu")).toBe(
       "brand new count: 3 · kihagyva: moon is full",
     );
   });

frontend/src/lib/jobResult.ts

@@ -22,16 +22,16 @@ export function parseSummary(summary: string | null | undefined): JobResult | nu
  *  stored "new=3, failed=0" — English keys, a Python dict repr for a nested phase. A count's key is
  *  named from `scheduler.result`, a `skipped` / `reason` word from `scheduler.skip`; an unnamed key or
  *  word shows as it is, and an `error` stays the upstream's own text (it is diagnostic). */
-export function jobResultText(t: TFunction, result: JobResult): string {
+export function jobResultText(t: TFunction, result: JobResult, lang: string): string {
   return Object.entries(result)
     .map(([key, value]) => {
       const label = t(`scheduler.result.${key}`, { defaultValue: key.replace(/_/g, " ") });
-      if (typeof value === "number") return `${label}: ${formatNumber(value)}`;
+      if (typeof value === "number") return `${label}: ${formatNumber(value, lang)}`;
       if (typeof value === "string")
         return key === "error"
           ? `${label}: ${value}`
           : `${label}: ${t(`scheduler.skip.${vocabKey(value)}`, { defaultValue: value })}`;
-      return `${label} (${jobResultText(t, value)})`;
+      return `${label} (${jobResultText(t, value, lang)})`;
     })
     .join(" · ");
 }

frontend/src/lib/localeFormat.contract.test.ts

@@ -2,18 +2,19 @@ import { describe, expect, it } from "vitest";
 
 import { eslint } from "./lintProbe.testkit";
 
-// `local/date-format` (eslint.config.js): docs/DESIGN.md "Copy" — a date is shown in the forms
-// lib/format.ts builds, in the UI language. This pins that the rule fires on the ways a date used to
-// be formatted by hand, and where it stands down.
-const RULE = "local/date-format";
+// `local/locale-format` (eslint.config.js): docs/DESIGN.md "Copy" — a date, a number and an
+// alphabetical order are in the UI language, built in lib/format.ts (text compared in lib/collate.ts),
+// and a component takes the language off useTranslation, never the i18n singleton. This pins that the
+// rule fires on the ways each used to be done by hand, and where it stands down.
+const RULE = "local/locale-format";
 
 async function hits(source: string, filePath = "src/components/__probe.tsx"): Promise<number> {
   const [result] = await eslint.lintText(source, { filePath });
   return (result?.messages ?? []).filter((m) => m.ruleId === RULE).length;
 }
 
-describe("a hand-formatted date", () => {
-  it("is refused as toLocale*String or an Intl date formatter", async () => {
+describe("a hand-formatted date or number", () => {
+  it("is refused as toLocale*String or an Intl date or number formatter", async () => {
     expect(await hits("export const a = (d: Date) => d.toLocaleString('en');\n")).toBe(1);
     expect(await hits("export const b = (d: Date) => d.toLocaleDateString();\n")).toBe(1);
     expect(await hits("export const c = (d: Date) => d.toLocaleTimeString();\n")).toBe(1);
@@ -21,12 +22,48 @@ describe("a hand-formatted date", () => {
       await hits("export const e = new Intl.DateTimeFormat('en', { month: 'short' });\n"),
     ).toBe(1);
     expect(await hits("export const f = new Intl.RelativeTimeFormat('en');\n")).toBe(1);
+    expect(await hits("export const h = new Intl.NumberFormat('en');\n")).toBe(1);
   });
 
-  it("is left alone in lib/format.ts, in a test and for a number format", async () => {
+  it("is left alone in lib/format.ts and in a test", async () => {
     const src = "export const g = (d: Date) => d.toLocaleString('en');\n";
     expect(await hits(src, "src/lib/format.ts")).toBe(0);
     expect(await hits(src, "src/lib/format.test.ts")).toBe(0);
-    expect(await hits("export const h = new Intl.NumberFormat('en');\n")).toBe(0);
+    expect(await hits("export const h = new Intl.NumberFormat('en');\n", "src/lib/format.ts")).toBe(
+      0,
+    );
+  });
+});
+
+describe("a hand-rolled alphabetical order", () => {
+  it("is refused as localeCompare, or an Intl.Collator outside lib/collate.ts", async () => {
+    expect(await hits("export const s = (a: string, b: string) => a.localeCompare(b);\n")).toBe(1);
+    expect(
+      await hits(
+        "export const s = (a: string, b: string) => a.localeCompare(b);\n",
+        "src/lib/x.ts",
+      ),
+    ).toBe(1);
+    expect(await hits("export const c = new Intl.Collator('hu');\n")).toBe(1);
+    expect(await hits("export const c = new Intl.Collator('hu');\n", "src/lib/collate.ts")).toBe(0);
+  });
+});
+
+describe("the language in a component", () => {
+  it("is refused off the i18n singleton", async () => {
+    const src = 'import i18n from "../i18n";\nexport const L = () => <b>{i18n.language}</b>;\n';
+    expect(await hits(src)).toBe(1);
+    const next =
+      'import i18next from "i18next";\nexport const L = () => <b>{i18next.language}</b>;\n';
+    expect(await hits(next)).toBe(1);
+  });
+
+  it("is taken from useTranslation, and the singleton stays usable outside components", async () => {
+    const hook =
+      'import { useTranslation } from "react-i18next";\n' +
+      "export const L = () => {\n  const { i18n } = useTranslation();\n  return <b>{i18n.language}</b>;\n};\n";
+    expect(await hits(hook)).toBe(0);
+    const lib = 'import i18n from "../i18n";\nexport const lang = () => i18n.language;\n';
+    expect(await hits(lib, "src/lib/__probe.ts")).toBe(0);
   });
 });

frontend/src/lib/playlistView.test.ts

@@ -46,17 +46,17 @@ describe("applyView", () => {
   ];
 
   it("returns the stored order untouched with no sort and no grouping", () => {
-    expect(applyView(rows, columns, null, false)).toBe(rows);
+    expect(applyView(rows, columns, null, false, "en")).toBe(rows);
   });
 
   it("sorts by the named column", () => {
-    expect(titles(applyView(rows, columns, { key: "title", dir: "asc" }, false))).toEqual([
+    expect(titles(applyView(rows, columns, { key: "title", dir: "asc" }, false, "en"))).toEqual([
       "alpha",
       "bravo",
       "charlie",
       "delta",
     ]);
-    expect(titles(applyView(rows, columns, { key: "title", dir: "desc" }, false))).toEqual([
+    expect(titles(applyView(rows, columns, { key: "title", dir: "desc" }, false, "en"))).toEqual([
       "delta",
       "charlie",
       "bravo",
@@ -65,7 +65,7 @@ describe("applyView", () => {
   });
 
   it("groups by channel, ordering the groups by name and sorting within each", () => {
-    expect(titles(applyView(rows, columns, { key: "title", dir: "asc" }, true))).toEqual([
+    expect(titles(applyView(rows, columns, { key: "title", dir: "asc" }, true, "en"))).toEqual([
       "alpha", // Alpha
       "bravo", // Alpha
       "charlie", // Beta
@@ -74,7 +74,7 @@ describe("applyView", () => {
   });
 
   it("reverses the GROUP order too when the sort is descending", () => {
-    expect(titles(applyView(rows, columns, { key: "title", dir: "desc" }, true))).toEqual([
+    expect(titles(applyView(rows, columns, { key: "title", dir: "desc" }, true, "en"))).toEqual([
       "delta", // Beta first, because the direction flips the group order as well
       "charlie",
       "bravo", // then Alpha
@@ -83,7 +83,7 @@ describe("applyView", () => {
   });
 
   it("groups without a sort by keeping each group's stored order", () => {
-    expect(titles(applyView(rows, columns, null, true))).toEqual([
+    expect(titles(applyView(rows, columns, null, true, "en"))).toEqual([
       "alpha",
       "bravo",
       "delta",
@@ -93,11 +93,9 @@ describe("applyView", () => {
 
   it("parks unknown durations at the end of an ascending sort, not at zero", () => {
     const mixed = [v("1", "long", "A", 900), v("2", "live", "A", null), v("3", "short", "A", 30)];
-    expect(titles(applyView(mixed, columns, { key: "duration", dir: "asc" }, false))).toEqual([
-      "short",
-      "long",
-      "live",
-    ]);
+    expect(titles(applyView(mixed, columns, { key: "duration", dir: "asc" }, false, "en"))).toEqual(
+      ["short", "long", "live"],
+    );
   });
 });
 

frontend/src/lib/playlistView.ts

@@ -1,4 +1,5 @@
 import { sortRows, type SortState } from "./columnSort";
+import { compareText } from "./collate";
 
 // The playlist detail's VIEW derivation: what the reader sees, as opposed to the playlist's stored
 // order. Kept structural (no `Column`, no component imports) like `columnSort.ts`, so it stays pure
@@ -21,8 +22,9 @@ export function applyView<T extends Grouped>(
   columns: Sortable<T>[],
   sort: SortState,
   group: boolean,
+  lang: string,
 ): T[] {
-  const sorted = sortRows(rows, columns, sort);
+  const sorted = sortRows(rows, columns, sort, lang);
   if (!group) return sorted;
   const groups = new Map<string, T[]>();
   for (const v of sorted) {
@@ -32,7 +34,7 @@ export function applyView<T extends Grouped>(
     else groups.set(k, [v]);
   }
   const sign = sort?.dir === "desc" ? -1 : 1;
-  const keys = [...groups.keys()].sort((a, b) => sign * a.localeCompare(b));
+  const keys = [...groups.keys()].sort((a, b) => sign * compareText(a, b, lang));
   return keys.flatMap((k) => groups.get(k)!);
 }
 

frontend/src/lib/plexPlayerHooks.ts

@@ -17,7 +17,7 @@ import {
 } from "./fullscreen";
 import { api, type PlexItemDetail, type PlexMarker } from "./api";
 import { qk } from "./queryKeys";
-import { plexLang } from "../i18n";
+import i18n, { plexLang } from "../i18n";
 import { type PlexPlayerPrefs } from "./plexPlayerPrefs";
 import { PLEX_SCALE, normalizeVolumeIn, stepVolumeIn } from "./playerVolume";
 
@@ -264,7 +264,7 @@ export function usePlexProgress(opts: {
     // checkpoint (the resume point when leaving).
     const settle = () => {
       // Match the language-keyed detail query (E5 S6) so the optimistic resume patch lands.
-      const key = qk.plexItem(id, plexLang());
+      const key = qk.plexItem(id, plexLang(i18n.language));
       if (finished) {
         // A refused finish claims nothing: the server has the item unwatched at the position the
         // rollback restored, and the catch below has already dropped this entry, so the next open
@@ -311,7 +311,7 @@ export function usePlexProgress(opts: {
         // `settle` may already have patched the detail to watched / 0 on the way out. Dropping that
         // entry rather than invalidating it: an invalidated query with no observer keeps serving its
         // stale data, and this one is a lie — no data is better, the next open fetches the truth.
-        qc.removeQueries({ queryKey: qk.plexItem(id, plexLang()) });
+        qc.removeQueries({ queryKey: qk.plexItem(id, plexLang(i18n.language)) });
       });
     };
     return () => {