import 'dart:async';
import 'dart:convert';

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

class AppLocalizations {
  final Locale locale;

  AppLocalizations(this.locale);

  static const LocalizationsDelegate<AppLocalizations> delegate =
      _AppLocalizationsDelegate();

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

  Map<String, String> _localizedStrings;

  Future<bool> load() async {
    // Load the language JSON file from the "lang" folder
    String jsonString =
        await rootBundle.loadString('lang/${locale.languageCode}.json');
    Map<String, dynamic> jsonMap = json.decode(jsonString);

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

    return true;
  }

  // This method will be called from every widget which needs a localized text
  String translate(String key) {
    return _localizedStrings[key];
  }
}

class _AppLocalizationsDelegate
    extends LocalizationsDelegate<AppLocalizations> {
  // This delegate instance will never change (it doesn't even have fields!)
  // It can provide a constant constructor.
  const _AppLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) {
    // Include all of your supported language codes here
    return ['en', 'id'].contains(locale.languageCode);
  }

  @override
  Future<AppLocalizations> load(Locale locale) async {
    // AppLocalizations class is where the JSON loading actually runs
    AppLocalizations localizations = new AppLocalizations(locale);
    await localizations.load();
    return localizations;
  }

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

import 'app_localizations.dart';
import 'app_localizations.dart';

...

return MaterialApp(
  title: 'Travel App',
  debugShowCheckedModeBanner: false,
  theme: ThemeData(
    primarySwatch: Colors.blue,
  ),
  supportedLocales: [
    Locale('en', 'US'),
    Locale('id', 'ID'),
  ],
  localizationsDelegates: [
    AppLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
  ],
  localeResolutionCallback: (locale, supportedLocales) {
    for (var supportedLocale in supportedLocales) {
      if (supportedLocale.languageCode == locale.languageCode &&
          supportedLocale.countryCode == locale.countryCode) {
        return supportedLocale;
      }
    }
    return supportedLocales.first;
  },
  home: MyHomePage(),
);
...

AppLocalizations.of(context).translate(key)

Text(AppLocalizations.of(context).translate('greeting'))