fix(messages): the welcome reads in the reader's language, not the one it was created in

E31 S1: the welcome is rendered once, at account creation, so an account that
switched language kept it in the old one (the owner's, English under a
Hungarian UI). A stored copy is recognised by its opening line (derived from
WELCOME) and served in the reader's language; other system messages stay as
stored.

backend/app/routes/messages.py

@@ -94,13 +94,30 @@ def _system_partner() -> dict:
     return {"id": SYSTEM_PARTNER_ID, "name": SYSTEM_NAME, "avatar_url": None}
 
 
-def _serialize_msg(m: Message) -> dict:
+# The welcome's opening line in every language — how a stored copy is recognised. It was rendered
+# once, in the language of the day it was created, so an account that later switched language kept
+# reading it in the old one (E31 S1: the owner's, in English). Recognised here, it is served in the
+# reader's language instead. Derived from WELCOME itself, so only a change to an OPENING line would
+# orphan the copies already stored.
+_WELCOME_OPENINGS = {text.partition("\n")[0] for text in WELCOME.values()}
+
+
+def _system_body(body: str | None, lang: str | None) -> str | None:
+    """A system message's text for a reader of `lang`: the welcome in their language, any other as
+    stored."""
+    if body is None or lang is None or body.partition("\n")[0] not in _WELCOME_OPENINGS:
+        return body
+    return WELCOME.get(lang, WELCOME["en"])
+
+
+def _serialize_msg(m: Message, lang: str | None = None) -> dict:
+    """`lang`: the reader's language, for a system message's text (a user message is ciphertext)."""
     return {
         "id": m.id,
         "kind": m.kind,
         "sender_id": m.sender_id,
         "recipient_id": m.recipient_id,
-        "body": m.body,  # only set for system messages
+        "body": _system_body(m.body, lang) if m.kind == "system" else m.body,
         "ciphertext": m.ciphertext,
         "iv": m.iv,
         "read_at": iso(m.read_at),
@@ -124,7 +141,7 @@ def ensure_welcome(db: Session, user: User) -> None:
     db.add(m)
     db.commit()
     db.refresh(m)
-    manager.push(user.id, {"type": "message", "message": _serialize_msg(m)})
+    manager.push(user.id, {"type": "message", "message": _serialize_msg(m, lang_for_user(user))})
 
 
 # --- E2EE key directory / blob store -----------------------------------------
@@ -313,7 +330,7 @@ def list_conversations(user: User = Depends(require_human), db: Session = Depend
         items.append(
             {
                 "partner": partner,
-                "last_message": _serialize_msg(m),
+                "last_message": _serialize_msg(m, lang_for_user(user)),
                 "unread": int(unread.get(pid, 0)),
             }
         )
@@ -366,7 +383,7 @@ def get_thread(
             # refreshes live instead of waiting for the next 20s poll. The client re-fetches the
             # count on this signal, so no number is carried here.
             manager.push(user.id, {"type": "unread"})
-    return {"partner": partner, "items": [_serialize_msg(m) for m in msgs]}
+    return {"partner": partner, "items": [_serialize_msg(m, lang_for_user(user)) for m in msgs]}
 
 
 @router.post("")

backend/tests/test_messages_welcome.py

@@ -0,0 +1,53 @@
+"""E31 S1: the welcome message is rendered once, when the account is created, in the language of that day.
+An account that switched language afterwards kept reading it in the old one (the owner's: English under a
+Hungarian UI). It is served in the reader's language now; any other system message stays as stored.
+Needs the DB."""
+
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import Message, User
+from app.routes.messages import WELCOME
+
+
+def _client(db, user, monkeypatch, override_dep) -> TestClient:
+    monkeypatch.setattr(state, "_configured_cache", True)
+    override_dep(get_db, db.info["request"])
+    override_dep(current_user, lambda: user)
+    return TestClient(app)
+
+
+def _user_with_english_welcome(db) -> User:
+    """An account whose welcome was written in English, then switched to Hungarian."""
+    user = User(email="welcome-hu@example.com", preferences={"language": "hu"})
+    db.add(user)
+    db.flush()
+    db.add(Message(kind="system", sender_id=None, recipient_id=user.id, body=WELCOME["en"]))
+    db.commit()
+    return user
+
+
+def test_the_welcome_is_read_in_the_readers_language(db, monkeypatch, override_dep):
+    user = _user_with_english_welcome(db)
+    client = _client(db, user, monkeypatch, override_dep)
+
+    thread = client.get("/api/messages/conversations/0").json()["items"]
+    assert [m["body"] for m in thread] == [WELCOME["hu"]]
+
+    convs = client.get("/api/messages/conversations").json()["items"]
+    assert [c["last_message"]["body"] for c in convs if c["partner"]["id"] == 0] == [WELCOME["hu"]]
+
+
+def test_another_system_message_is_served_as_stored(db, monkeypatch, override_dep):
+    user = _user_with_english_welcome(db)
+    db.add(
+        Message(kind="system", sender_id=None, recipient_id=user.id, body="Maintenance tonight.")
+    )
+    db.commit()
+    client = _client(db, user, monkeypatch, override_dep)
+
+    bodies = [m["body"] for m in client.get("/api/messages/conversations/0").json()["items"]]
+    assert bodies == [WELCOME["hu"], "Maintenance tonight."]