Example of overriding default locale for use with intl

intl_locale_override.dart

import 'dart:async';
import 'package:intl/intl.dart';
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:intl_translate_example/l10n/messages_all.dart';

void main() => runApp(new MyApp());

class Translations {
  static Future<Translations> load(Locale locale) {
    final String name =
        locale.countryCode.isEmpty ? locale.languageCode : locale.toString();
    final String localeName = Intl.canonicalizedLocale(name);

    return initializeMessages(localeName).then((bool _) {
      Intl.defaultLocale = localeName;
      return new Translations();
    });
  }

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

  String get title2 {
    return Intl.message(
      'Hello World',
      name: 'title3',
      desc: 'Title for the Demo application',
    );
  }
}

class TranslationsDelegate extends LocalizationsDelegate<Translations> {
  const TranslationsDelegate();

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

  @override
  Future<Translations> load(Locale locale) => Translations.load(locale);

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

class SpecifiedLocalizationDelegate
    extends LocalizationsDelegate<Translations> {
  final Locale overriddenLocale;

  const SpecifiedLocalizationDelegate(this.overriddenLocale);

  @override
  bool isSupported(Locale locale) => overriddenLocale != null;

  @override
  Future<Translations> load(Locale locale) =>
      Translations.load(overriddenLocale);

  @override
  bool shouldReload(SpecifiedLocalizationDelegate old) => true;
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {
  SpecifiedLocalizationDelegate _localeOverrideDelegate;

  @override
  void initState() {
    super.initState();
    _localeOverrideDelegate = new SpecifiedLocalizationDelegate(null);
  }

  onLocaleChange(Locale l) {
    setState(() {
      _localeOverrideDelegate = new SpecifiedLocalizationDelegate(l);
    });
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      theme: new ThemeData(primarySwatch: Colors.blue),
      localizationsDelegates: [
        _localeOverrideDelegate,
        const TranslationsDelegate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: [
        const Locale('en', ''), // English
        const Locale('fr', ''), // French
      ],
      home: new MyHomePage(onLocaleChange: onLocaleChange),
    );
  }
}

typedef void LocaleChangeCallback(Locale locale);

class MyHomePage extends StatefulWidget {
  final LocaleChangeCallback onLocaleChange;

  MyHomePage({Key key, this.onLocaleChange}) : super(key: key);

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(Translations.of(context).title2),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new RaisedButton(
              onPressed: () => widget.onLocaleChange(const Locale("en", "")),
              child: new Text("EN"),
            ),
            new RaisedButton(
              onPressed: () => widget.onLocaleChange(const Locale("fr", "")),
              child: new Text("FR"),
            ),
            new RaisedButton(
              onPressed: () => widget.onLocaleChange(null),
              child: new Text("Default"),
            ),
          ],
        ),
      ),
    );
  }
}

where is messages_all.dart?

and I have error initializeMessages isn't defined for the class Trahslations

Not working

where is messages_all.dart?

and I have error initializeMessages isn't defined for the class Trahslations

it will be generated automatically see documentation https://flutter.dev/docs/development/accessibility-and-localization/internationalization
and run I will try to publish some article on How to do it with a efficient way to change locale in the run time

//flutter pub pub run intl_translation:generate_from_arb --output-dir=lib/l10n --no-use-deferred-loading  lib/l10n/intl_en.arb lib/l10n/intl_messages.arb lib/l10n/intl_kn.arb lib/locale/locale.dart

Not working

can you try with ChangeNotifierProvider its so simple to use

Not working

may be my answer here can help you https://stackoverflow.com/a/59410830/8253662