在Flutter应用中实现国际化（i18n）是一个常见的需求，flutter_localizations 包和 intl 包为我们提供了强大的支持。下面是一个使用 flutter_localizations 和 intl 包实现国际化的代码案例。

1. 添加依赖

首先，在 pubspec.yaml 文件中添加必要的依赖：

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: ^0.17.0 # 确保使用最新版本

2. 创建语言文件

在你的项目根目录下创建一个 lib/l10n 文件夹，并在其中创建多个语言文件，例如 messages_en.dart 和 messages_zh.dart。

messages_en.dart

import 'package:intl/intl.dart';

final Map<String, Map<String, String>> enTranslations = {
  'app_title': {
    'title': 'My Flutter App',
  },
  'greeting': {
    'hello': 'Hello!',
  },
};

class MessagesEn extends IntlLocale {
  const MessagesEn() : super('en');

  static MessagesEn of(BuildContext context) {
    return Localizations.of<MessagesEn>(context, MessagesEn)!;
  }

  String get appTitle => Intl.message(
    'My Flutter App',
    name: 'app_title_title',
    desc: 'Title of the application',
    locale: localeName,
  );

  String get hello => Intl.message(
    'Hello!',
    name: 'greeting_hello',
    desc: 'A greeting message',
    locale: localeName,
  );
}

messages_zh.dart

import 'package:intl/intl.dart';

final Map<String, Map<String, String>> zhTranslations = {
  'app_title': {
    'title': '我的Flutter应用',
  },
  'greeting': {
    'hello': '你好！',
  },
};

class MessagesZh extends IntlLocale {
  const MessagesZh() : super('zh');

  static MessagesZh of(BuildContext context) {
    return Localizations.of<MessagesZh>(context, MessagesZh)!;
  }

  String get appTitle => Intl.message(
    '我的Flutter应用',
    name: 'app_title_title',
    desc: '应用程序的标题',
    locale: localeName,
  );

  String get hello => Intl.message(
    '你好！',
    name: 'greeting_hello',
    desc: '一个问候语',
    locale: localeName,
  );
}

3. 创建 localizations_delegates.dart

在 lib/l10n 文件夹中创建一个 localizations_delegates.dart 文件，用于定义本地化委托。

import 'package:flutter/material.dart';
import 'messages_en.dart';
import 'messages_zh.dart';

class MyLocalizationsDelegate extends LocalizationsDelegate<Locale> {
  const MyLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) {
    return locale.languageCode == 'en' || locale.languageCode == 'zh';
  }

  @override
  Future<Locale> load(Locale locale) async {
    return locale;
  }

  @override
  bool shouldReload(MyLocalizationsDelegate oldDelegate) {
    return false;
  }
}

class MyLocalizations {
  MyLocalizations(this.locale);

  final Locale locale;

  static MyLocalizations of(BuildContext context) {
    return Localizations.of<MyLocalizations>(context, MyLocalizations)!;
  }

  MessagesEn get en => MessagesEn.of(context);
  MessagesZh get zh => MessagesZh.of(context);

  String get appTitle {
    if (locale.languageCode == 'zh') {
      return zh.appTitle;
    } else {
      return en.appTitle;
    }
  }

  String get hello {
    if (locale.languageCode == 'zh') {
      return zh.hello;
    } else {
      return en.hello;
    }
  }
}

4. 在应用中使用本地化

在 MaterialApp 中设置 localizationsDelegates 和 supportedLocales。

import 'package:flutter/material.dart';
import 'lib/l10n/localizations_delegates.dart';
import 'lib/l10n/messages_en.dart';
import 'lib/l10n/messages_zh.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      localizationsDelegates: [
        MyLocalizationsDelegate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
        GlobalCupertinoLocalizations.delegate,
      ],
      supportedLocales: [
        Locale('en', ''),
        Locale('zh', ''),
      ],
      locale: Locale('en'), // 默认语言
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final MyLocalizations localizations = MyLocalizations.of(context);

    return Scaffold(
      appBar: AppBar(
        title: Text(localizations.appTitle),
      ),
      body: Center(
        child: Text(localizations.hello),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          // 切换语言示例（这里仅为演示，实际使用中可能需要持久化用户选择的语言）
          Navigator.push(
            context,
            MaterialPageRoute<void>(
              builder: (BuildContext context) {
                return LanguageSelectorPage(currentLocale: Localizations.localeOf(context));
              },
            ),
          );
        },
        tooltip: 'Change Language',
        child: Icon(Icons.language),
      ),
    );
  }
}

class LanguageSelectorPage extends StatefulWidget {
  final Locale currentLocale;

  const LanguageSelectorPage({Key? key, required this.currentLocale}) : super(key: key);

  @override
  _LanguageSelectorPageState createState() => _LanguageSelectorPageState();
}

class _LanguageSelectorPageState extends State<LanguageSelectorPage> {
  Locale? _selectedLocale;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Select Language'),
      ),
      body: ListView(
        children: [
          ListTile(
            title: Text('English'),
            leading: Icon(Icons.language),
            onTap: () {
              setState(() {
                _selectedLocale = Locale('en');
                Navigator.popAndPushNamed(context, '/');
              });
            },
          ),
          ListTile(
            title: Text('中文'),
            leading: Icon(Icons.language),
            onTap: () {
              setState(() {
                _selectedLocale = Locale('zh');
                Navigator.popAndPushNamed(context, '/');
              });
            },
          ),
        ],
      ),
    );
  }
}

5. 运行应用

运行你的Flutter应用，你应该能看到默认显示英文标题和问候语。点击浮动按钮（FAB）可以切换到语言选择页面，选择中文后应用会重启并显示中文标题和问候语。

这个示例展示了如何使用 flutter_localizations 和 intl 包来实现Flutter应用的国际化。你可以根据需要扩展语言文件和本地化逻辑。