Validate locale override and use preferred locale list for fallback

lib/screens/channel_chat_screen.dart

@@ -57,6 +57,10 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
   DateTime? _lastChannelSendAt;
   bool _channelSkipNextBottomSnap = false;
 
+  String? _cachedFormatLocale;
+  late DateFormat _hmFormat;
+  late DateFormat _mdFormat;
+
   @override
   void initState() {
     super.initState();
@@ -1232,10 +1236,15 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
     final now = DateTime.now();
     final diff = now.difference(time);
     final locale = Localizations.localeOf(context).toString();
-    final hm = DateFormat.Hm(locale).format(time);
+    if (locale != _cachedFormatLocale) {
+      _cachedFormatLocale = locale;
+      _hmFormat = DateFormat.Hm(locale);
+      _mdFormat = DateFormat.Md(locale);
+    }
+    final hm = _hmFormat.format(time);
 
     if (diff.inDays > 0) {
-      return '${DateFormat.Md(locale).format(time)} $hm';
+      return '${_mdFormat.format(time)} $hm';
     } else {
       return hm;
     }

lib/services/background_service.dart

@@ -57,14 +57,17 @@ class BackgroundService {
     final supported = AppLocalizations.supportedLocales;
     final override = _languageOverrideProvider?.call();
     if (override != null && override.isNotEmpty) {
-      return AppLocalizations.delegate.load(Locale(override));
+      final overrideLocale = Locale(override);
+      final isSupported = supported.any(
+        (l) => l.languageCode == overrideLocale.languageCode,
+      );
+      if (isSupported) {
+        return AppLocalizations.delegate.load(overrideLocale);
+      }
     }
-    final system =
-        WidgetsBinding.instance.platformDispatcher.locale;
-    final match = basicLocaleListResolution(
-      <Locale>[system],
-      supported,
-    );
+    final preferred =
+        WidgetsBinding.instance.platformDispatcher.locales;
+    final match = basicLocaleListResolution(preferred, supported);
     return AppLocalizations.delegate.load(match);
   }