需求

一個app 最少也有兩種語言 英文和中文 ，但在flutter 要如何實作呢?
那就來看一下 intl 如何用吧

intl - Readme

intl 除了國際化和在地化，還包括訊息翻譯、複數和性別、日期/數字格式和解析以及雙向文字。

Intl.defaultLocale = 'pt_BR'; //set the global locale

Intl.defaultLocale = 'es';
DateFormat.jm().format(DateTime.now());

intl - Installing

直接在 pubspec.yaml 加上 ntl: ^0.18.0 ，然後pub get 

dependencies:
  intl: ^0.18.0  

但 intl 有相依性，要再加上 flutter_localizations

dependencies:
    flutter_localizations:
    sdk: flutter

intl - Example

intl 的 package info 沒有 Example 大家要參考官網的說明或搜尋一下

1. 建立語言資源文件
   建立 en.json和 zh.json 檔，放在 path/locales 目錄中

en.json

{
  "Day12 intl": "Day12 intl internationalization and localization"
}   

zh.json

{
  "Day12 intl": "Day12 intl 國際化和在地化"
}

2. 在pubspec.yaml中宣告資源

flutter:
  assets:
    - lib/l10n/en.json
    - lib/l10n/zh.json

3. 加上 AppLocalizations class

class AppLocalizations {
  final Locale locale;
  Map<String, String>? _localizedStrings;

  AppLocalizations(this.locale);

  static AppLocalizations? of(BuildContext context) {
    return Localizations.of<AppLocalizations>(context, AppLocalizations);
  }

  Future<bool> load() async {
    String jsonString = await rootBundle.loadString('lib/l10n/${locale.languageCode}.json');
    Map<String, dynamic> jsonMap = json.decode(jsonString);

    _localizedStrings = jsonMap.map((key, value) {
      return MapEntry(key, value.toString());
    });

    return true;
  }

  String translate(String key) {
    return _localizedStrings?[key] ?? 'Missing translation';
  }
}

4. 實作 LocalizationsDelegate

class AppLocalizationsDelegate extends LocalizationsDelegate<AppLocalizations> {
  const AppLocalizationsDelegate();

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

  @override
  Future<AppLocalizations> load(Locale locale) async {
    AppLocalizations localizations = new AppLocalizations(locale);
    await localizations.load();
    return localizations;
  }

  @override
  bool shouldReload(AppLocalizationsDelegate old) => false;
}

5. 配置MaterialApp

return MaterialApp(
...
localizationsDelegates: [
        AppLocalizationsDelegate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: [
        Locale('en', 'US'),
        Locale('zh', 'TW'),
      ],
      localeResolutionCallback: (locale, supportedLocales) {
        if (locale == null) {
          return supportedLocales.first;
        }
        for (var supportedLocale in supportedLocales) {
          if (supportedLocale.languageCode == locale.languageCode &&
              supportedLocale.countryCode == locale.countryCode) {
            return supportedLocale;
          }
        }
        return supportedLocales.first;
      },
...      

6. 多語系使用方式

Text(AppLocalizations.of(context)?.translate('Day12 intl')?? "data"),

執行結果

intl zh 中文

intl en 英文

心得

intl 使用起來相對沒有前面幾天介紹的簡單，有遇到要相依套件、文件沒有範例，
一一克服後，還算滿好用的

參考

https://docs.flutter.dev/ui/accessibility-and-localization/internationalization