feat(frontend): add multi-language support (en, zh-Hans, zh-Hant, ja)

frontend/lib/main.dart

@@ -1,10 +1,12 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:provider/provider.dart';
 import 'package:fvp/fvp.dart' as fvp;
 import 'providers/auth_provider.dart';
 import 'providers/settings_provider.dart';
 import 'pages/home_page.dart';
 import 'pages/login_page.dart';
+import 'l10n/app_localizations.dart';
 
 void main() {
   fvp.registerWith();
@@ -30,6 +32,14 @@ class HightubeApp extends StatelessWidget {
 
     return MaterialApp(
       title: 'Hightube',
+      localizationsDelegates: const [
+        AppLocalizations.delegate,
+        GlobalMaterialLocalizations.delegate,
+        GlobalWidgetsLocalizations.delegate,
+        GlobalCupertinoLocalizations.delegate,
+      ],
+      supportedLocales: AppLocalizations.supportedLocales,
+      locale: settings.locale,
       theme: ThemeData(
         useMaterial3: true,
         colorScheme: ColorScheme.fromSeed(