feat: language setting

lib/main.dart

@@ -45,12 +45,15 @@ class _MainAppState extends State<MainApp> {
       SharedPrefSingleton().appTheme ? ThemeMode.light : ThemeMode.dark;
   ThemeMode get getCurrentTheme => _themeMode;
 
+  Locale? _locale;
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       theme: ThemeData(),
       darkTheme: ThemeData.dark(),
       themeMode: _themeMode,
+      locale: _locale,
       localizationsDelegates: AppLocalizations.localizationsDelegates,
       supportedLocales: AppLocalizations.supportedLocales,
       home: const Scaffold(
@@ -59,6 +62,12 @@ class _MainAppState extends State<MainApp> {
     );
   }
 
+  void changeLocale(Locale locale) {
+    setState(() {
+      _locale = locale;
+    });
+  }
+
   void changeTheme(ThemeMode mode) {
     mode == ThemeMode.light
         ? SharedPrefSingleton().setAppTheme(true).then((onValue) {

lib/setting_page.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:winui_n2n/main.dart';
 import 'package:winui_n2n/shared_pref_singleton.dart';
 import 'package:flutter_gen/gen_l10n/app_localizations.dart';
 
@@ -30,6 +31,23 @@ class _SettingPageState extends State<SettingPage> {
             ),
           ],
         ),
+        DropdownButton<Locale>(
+          value: Localizations.localeOf(context),
+          items:
+              AppLocalizations.supportedLocales.map<DropdownMenuItem<Locale>>(
+            (e) {
+              return DropdownMenuItem(
+                value: e,
+                child: Text(e.toString()),
+              );
+            },
+          ).toList(),
+          onChanged: (value) {
+            setState(() {
+              MainApp.of(context).changeLocale(value!);
+            });
+          },
+        ),
         ElevatedButton(
           onPressed: () {
             ScaffoldMessenger.of(context)