feat(i18n): retire the German interface language

Only English and Hungarian remain. Drops the de locale files, the LANGUAGES
entry, the Google-locale mapping in auth, and the German welcome-message
template. A legacy "de" preference falls back to English gracefully. Content-
language detection and subtitle languages are untouched (media, not UI).

backend/app/auth.py

@@ -364,7 +364,7 @@ async def callback(
     prefs = dict(user.preferences or {})
     if not prefs.get("language"):
         loc = (userinfo.get("locale") or "").split("-")[0].lower()
-        prefs["language"] = loc if loc in {"en", "hu", "de"} else "en"
+        prefs["language"] = loc if loc in {"en", "hu"} else "en"
         user.preferences = prefs
     db.flush()