fix(watch): a link whose video is being made again says so, in the page's language

The public watch route answers video_preparing for an asset that is pending or
downloading, and the page shows "being prepared — try again" with a Retry
instead of "this link is gone". watch.html ships one lang; the page now sets
documentElement.lang to the language its strings were picked in.

frontend/src/components/watchStrings.dom.test.ts

@@ -0,0 +1,30 @@
+// @vitest-environment jsdom
+//
+// The shared-link page ships one watch.html for every visitor, so the document's language is set
+// from the language its strings were picked in — a screen reader picks its voice by it, and the
+// browser offers to translate by it. A Hungarian visitor once got lang="en" over Hungarian text.
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  vi.resetModules();
+});
+
+async function pageIn(language: string) {
+  vi.stubGlobal("navigator", { ...navigator, language });
+  vi.resetModules();
+  return import("./watchStrings");
+}
+
+describe("the watch page's document language", () => {
+  it.each([
+    ["hu-HU", "hu"],
+    ["de-AT", "de"],
+    ["en-GB", "en"],
+    ["fr-FR", "en"], // no French strings: the page is in English, and says so
+  ])("a %s browser reads a page marked %s", async (language, want) => {
+    const { lang } = await pageIn(language);
+    expect(lang).toBe(want);
+    expect(document.documentElement.lang).toBe(want);
+  });
+});