一、导包

引入“flutter_localizations包”，并且“flutter packages get”：

dependencies:

  ...

  flutter_localizations:

    sdk: flutter

二、添加协议以及支持语言

import 'package:flutter_localizations/flutter_localizations.dart';
...
return MaterialApp(
      ...
      localizationsDelegates: [
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: [
        const Locale('zh', 'CN'),
        const Locale('zh', 'HK'),
        const Locale('zh', 'TW'),
        const Locale('en', 'US'),
      ],
    );//MaterialApp

三、编写Localizations类

Localizations汇总所有翻译文本

import 'package:flutter/material.dart';

class  MyLocalizations {
  final Locale locale;

  MyLocalizations(this.locale);

  static Map<String, Map<String, String>> _localizedValuesMap = {
    'en_US': {
      'test_text': 'Test Text',
    },
    'zh_CN': {
      'test_text': '测试文字',
    },
    'zh_HK': {
      'test_text': '測試文字',
    },
    'zh_TW': {
      'test_text': '測試文字',
    }
  };

  get testText{
    ///注意用 locale.toString()而非locale
    return _localizedValuesMap[locale.toString()]['test_text'];
  }

  ///MyLocalizations的实例是在 Localizations中通过 MyLocalizationsDelegate实例化,
  ///应用中要使用 MyLocalizations的实例需要通过 Localizations这个 Widget来获取。
  static MyLocalizations of(BuildContext context){
    return Localizations.of(context, MyLocalizations);
  }
}

四、实现LocalizationsDelegate协议，用于初始化Localizations类

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:gd_i18n_project/MyLocalizations.dart';

class MyLocalizationsDelegate extends LocalizationsDelegate<MyLocalizations>{
  const MyLocalizationsDelegate();
  static MyLocalizationsDelegate delegate = const MyLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) {
    return ['en','zh'].contains(locale.languageCode);
  }

  @override
  Future<MyLocalizations> load(Locale locale) {
    ///这里初始化 Localizations类
    return SynchronousFuture<MyLocalizations>(new MyLocalizations(locale));
  }

  @override
  bool shouldReload(LocalizationsDelegate<MyLocalizations> old) {
    return false;
  }
}

添加到"localizationsDelegates":

return MaterialApp(
      ...
      localizationsDelegates: [
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
        ///自定义 LocalizationsDelegate协议；
        MyLocalizationsDelegate(),
      ],
      ...
    );///MaterialApp

五、替换应用的Title

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      onGenerateTitle: (context){
        //默认的 build()的 context对象无法获得 MyLocalizations对象，
        //所以这里引用 onGenerateTitle的 context对象。
        return MyLocalizations.of(context).testText;
      },
...

六、替换MyHomePage的Title

 Widget build(BuildContext context) {
    return MaterialApp(
      ...
      //修改MyHomePage()的构造函数，去掉title
      home: MyHomePage(),
      ...

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        //同第五步，默认的 build()的 context对象无法获得 MyLocalizations对象。
        title: Text(MyLocalizations.of(context).testText),
      ),
      ...

到这一步应用能跟随系统更改语言。

七、运行时更改语言

import 'package:flutter/material.dart';

///用于操作子widget运行时更改语言
class MyI18nWidget extends StatefulWidget {
  final child;
  MyI18nWidget({this.child, Key key}):super(key:key);

  @override
  State<StatefulWidget> createState() => MyI18nWidgetState();
}

class MyI18nWidgetState extends State<MyI18nWidget> {
  var _locale;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    //获取当前设备语言
    _locale = Localizations.localeOf(context);
  }

  ///动态切换子widget的语言
  void changeLanguage(Locale locale){
    setState(() {
      _locale=locale;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Localizations.override(
      context: context,
      locale: _locale,
      child: widget.child,
    );
  }
}

void main() => runApp(MyApp());
///用于外部获取MyI18nWidgetState来调用其changeLanguage();
GlobalKey<MyI18nWidgetState> myI18nWidgetStateKey=GlobalKey<MyI18nWidgetState>();
class MyApp extends StatelessWidget {
...
}

return MaterialApp(
      ...
      //MyI18nWidget作为MyHomePage的父widget
      home: MyI18nWidget(
        key: myI18nWidgetStateKey,
        child: MyHomePage(
        ),
      ),
      ...
)//MaterialApp

//指定地方调用，动态更改语言
myI18nWidgetStateKey.currentState.changeLanguage(Locale('en','US'));

参考

《Flutter 中的国际化》_番茄沙司 https://www.jianshu.com/p/8356a3bc8f6c