如文件所示：
要提供本地化标题，请使用onGenerateTitle

试试这个
main.dart:

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

import 'config/languages/localizations.dart';
import 'config/languages/localizations_delegate.dart';
import 'homepage.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) => MaterialApp(
        debugShowCheckedModeBanner: false,
        localizationsDelegates: const [
          MyLocalizationsDelegate(),
          GlobalMaterialLocalizations.delegate,
          GlobalWidgetsLocalizations.delegate,
          GlobalCupertinoLocalizations.delegate,
        ],
        supportedLocales: const [
          Locale('en', ''),
          Locale('de', ''),
        ],
        onGenerateTitle: (context) => MyLocalizations.of(context)?.title ?? '',
        theme: ThemeData(primarySwatch: Colors.blue),
        home: const MyHomePage(),
      );
}

localizations.dart:

import 'package:flutter/material.dart';

class MyLocalizations {
  MyLocalizations(this.locale);

  final Locale locale;

  static MyLocalizations? of(BuildContext context) =>
      Localizations.of<MyLocalizations>(context, MyLocalizations);

  static languages() => _localizedValues.keys.toList();

  String get title => _localizedValues[locale.languageCode]!['title']!;

  static const _localizedValues = <String, Map<String, String>>{
    'en': {
      'title': 'Hello World',
    },
    'de': {
      'title': 'Hallo Welt',
    },
  };
}

本地化_委托.dart：

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

import 'localizations.dart';

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

  @override
  bool isSupported(Locale locale) =>
      MyLocalizations.languages().contains(locale.languageCode);

  @override
  Future<MyLocalizations> load(Locale locale) =>
      SynchronousFuture<MyLocalizations>(MyLocalizations(locale));

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