App.dart

Future main() async {
  final Iterable<Locale> supportedLocales = [Locale('en'), Locale('ar')];
  await Langs.init(supportedLocales: supportedLocales, loadDeviceLanguage: true);
  runApp(MyApp(supportedLocales: supportedLocales));
  print(Langs.string('sample.item.name'));
  print(Langs.string('name'));
}

en.json

{
  "name": "Hello World!",
  "sample": {
    "item": {
      "name": "Hello Nested World!"
    }
  }
}

Langs.dart

import 'dart:convert';
import 'dart:ui';
import 'dart:ui' as ui;

import 'package:flutter/cupertino.dart';
import 'package:flutter/services.dart' show rootBundle;

class Langs {
  static Map<String, dynamic> _currentLanguage;
  static Iterable<Locale> _supportedLocales;
  static Locale _defaultLocale;

  static Future init(
      {Locale defaultLocale = const Locale('en'),
      Iterable<Locale> supportedLocales = const [Locale('en')],
      bool loadDeviceLanguage = false}) async {
    _defaultLocale = defaultLocale;
    _supportedLocales = supportedLocales;
    if (loadDeviceLanguage) {
      await Langs.loadDeviceLanguage();
    }
  }

  static Future loadDeviceLanguage() async {
    final deviceLocale = ui.window.locale;
    if (_supportedLocales.contains(deviceLocale)) {
      await setLang(deviceLocale.toLanguageTag());
    } else {
      if (_supportedLocales
          .map((l) => l.languageCode)
          .contains(deviceLocale.languageCode)) {
        await setLang(deviceLocale.languageCode);
      } else {
        await setLang(_defaultLocale.toLanguageTag());
      }
    }
  }

  static Future setLang(String lang) async {
    _currentLanguage =
        json.decode(await rootBundle.loadString("resources/langs/$lang.json"));
  }

  static string(String id) {
    if (id.contains('.')) {
      return id.split('.').fold(_currentLanguage, (map, key) {
        return map[key];
      });
    } else {
      if (_currentLanguage.containsKey(id)) {
        return _currentLanguage[id];
      }
      throw Exception("No translation found for key: $id");
    }
  }
}