1.编写自己的Delegate

import 'package:flutter/foundation.dart' show SynchronousFuture;
import 'package:flutter/cupertino.dart';

class CupertinoLocalizationsDelegate
    extends LocalizationsDelegate<CupertinoLocalizations> {
  const CupertinoLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) =>
      <String>['en', 'zh'].contains(locale.languageCode);

  @override
  SynchronousFuture<_DefaultCupertinoLocalizations> load(Locale locale) {
    return SynchronousFuture<_DefaultCupertinoLocalizations>(
        _DefaultCupertinoLocalizations(locale.languageCode));
  }

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

class _DefaultCupertinoLocalizations extends CupertinoLocalizations {
  _DefaultCupertinoLocalizations(this._languageCode)
      : assert(_languageCode != null);

  final DefaultCupertinoLocalizations _en =
      const DefaultCupertinoLocalizations();
  final String _languageCode;

  final Map<String, Map<String, String>> _dict = <String, Map<String, String>>{
    'en': <String, String>{
      'alert': 'Alert',
      'copy': 'Copy',
      'paste': 'Paste',
      'cut': 'Cut',
      'selectAll': 'Select all',
      'today': 'today'
    },
    'zh': <String, String>{
      'alert': '警告',
      'copy': '复制',
      'paste': '粘贴',
      'cut': '剪切',
      'selectAll': '选择全部',
      'today': '今天'
    }
  };

  @override
  String get alertDialogLabel => _get('alert');

  @override
  String get anteMeridiemAbbreviation => _en.anteMeridiemAbbreviation;

  @override
  String get postMeridiemAbbreviation => _en.postMeridiemAbbreviation;

  @override
  String get copyButtonLabel => _get('copy');

  @override
  String get cutButtonLabel => _get('cut');

  @override
  String get pasteButtonLabel => _get('paste');

  @override
  String get selectAllButtonLabel => _get('selectAll');

  @override
  DatePickerDateOrder get datePickerDateOrder => _en.datePickerDateOrder;

  @override
  DatePickerDateTimeOrder get datePickerDateTimeOrder =>
      _en.datePickerDateTimeOrder;

  @override
  String datePickerDayOfMonth(int dayIndex) =>
      _en.datePickerDayOfMonth(dayIndex);

  @override
  String datePickerHour(int hour) => _en.datePickerHour(hour);

  @override
  String datePickerHourSemanticsLabel(int hour) =>
      _en.datePickerHourSemanticsLabel(hour);

  @override
  String datePickerMediumDate(DateTime date) => _en.datePickerMediumDate(date);

  @override
  String datePickerMinute(int minute) => _en.datePickerMinute(minute);

  @override
  String datePickerMinuteSemanticsLabel(int minute) =>
      _en.datePickerMinuteSemanticsLabel(minute);

  @override
  String datePickerMonth(int monthIndex) => _en.datePickerMonth(monthIndex);

  @override
  String datePickerYear(int yearIndex) => _en.datePickerYear(yearIndex);

  @override
  String timerPickerHour(int hour) => _en.timerPickerHour(hour);

  @override
  String timerPickerHourLabel(int hour) => _en.timerPickerHourLabel(hour);

  @override
  String timerPickerMinute(int minute) => _en.timerPickerMinute(minute);

  @override
  String timerPickerMinuteLabel(int minute) =>
      _en.timerPickerMinuteLabel(minute);

  @override
  String timerPickerSecond(int second) => _en.timerPickerSecond(second);

  @override
  String timerPickerSecondLabel(int second) =>
      _en.timerPickerSecondLabel(second);

  String _get(String key) {
    return _dict[_languageCode][key];
  }

  @override
  String get todayLabel => _get("today");
}

2.在main.dart中引入

localizationsDelegates: [
          //此处
          CupertinoLocalizationsDelegate(),
          GlobalMaterialLocalizations.delegate,
          GlobalWidgetsLocalizations.delegate,
        ],
        supportedLocales: [
          const Locale('zh'),
          const Locale('en')
        ]);