输入框长按选项菜单复制、粘贴、剪切、全选部分默认为英文，对于只需要对此部分做中文本地化，不需要考虑其他语言及全局本地化的项目，可以直接自定义一个本地化代理方法进行覆盖，不需要额外下载插件

// 自定义本地化代理
class _ChineseLocalizationsDelegate
    extends LocalizationsDelegate<MaterialLocalizations> {
  const _ChineseLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) => true;

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

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

// 自定义 MaterialLocalizations
class _ChineseMaterialLocalizations extends DefaultMaterialLocalizations {
  const _ChineseMaterialLocalizations();

  @override
  String get copyButtonLabel => '复制';
  @override
  String get pasteButtonLabel => '粘贴';
  @override
  String get cutButtonLabel => '剪切';
  @override
  String get selectAllButtonLabel => '全选';
}

然后将其添加到main函数中的主app中

return MaterialApp(
    locale: Locale('zh', 'CH'),
    localizationsDelegates: const [
    _ChineseLocalizationsDelegate(),
    DefaultWidgetsLocalizations.delegate,
    DefaultMaterialLocalizations.delegate,
    ],
    supportedLocales: [
      const Locale('zh', 'CH'),
    ],
  );