Simple app localization with JSON as input format and access without BuildContext. Used on https://www.flutterclutter.dev/flutter/tutorials/internationalization-i18n-in-flutter-an-easy-and-quick-approach/2020/213/

app_localizations.dart

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

class AppLocalizations {
  AppLocalizations(this.locale);
  final Locale fallbackLocale = Locale('en');

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

  static AppLocalizations instance;

  AppLocalizations._init(Locale locale) {
    instance = this;
    this.locale = locale;
  }

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

  Locale locale;
  Map<String, String> _localizedStrings;
  Map<String, String> _fallbackLocalizedStrings;

  Future<void> load() async {
    _localizedStrings = await _loadLocalizedStrings(locale);
    _fallbackLocalizedStrings = {};

    if (locale != fallbackLocale) {
      _fallbackLocalizedStrings = await _loadLocalizedStrings(fallbackLocale);
    }
  }

  Future<Map<String, String>> _loadLocalizedStrings(Locale localeToBeLoaded) async {
    String jsonString;
    Map<String, String> localizedStrings = {};

    try {
      jsonString = await rootBundle.loadString('assets/translations/${localeToBeLoaded.languageCode}.json');
    } catch (exception) {
      print(exception);
      return localizedStrings;
    }

    Map<String, dynamic> jsonMap = json.decode(jsonString);

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

    return localizedStrings;
  }

  String translate(String key, [Map<String, String> arguments]) {
    String translation = _localizedStrings[key];
    translation = translation ?? _fallbackLocalizedStrings[key];
    translation = translation ?? "";

    if (arguments == null || arguments.length == 0) {
      return translation;
    }

    arguments.forEach((argumentKey, value) {
      if (value == null) {
        print('Value for "$argumentKey" is null in call of translate(\'$key\')');
        value = '';
      }
      translation = translation.replaceAll("\$$argumentKey", value);
    });

    return translation;
  }
}

class _AppLocalizationsDelegate extends LocalizationsDelegate<AppLocalizations> {
  const _AppLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) {
    return true;
  }

  @override
  Future<AppLocalizations> load(Locale locale) async {
    AppLocalizations localizations = AppLocalizations._init(locale);
    await localizations.load();
    return localizations;
  }

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

de.json

{
  "string": "Beeindruckend!",
  "string_with_interpolation": "Hey $name, wie geht es dir?"
}

en.json

{
  "string": "Impressive!",
  "string_with_interpolation": "Hey $name, how are you?"
}

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:flutterclutter/app_localizations.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      supportedLocales: [
        const Locale('en', 'US'),
        const Locale('de', 'DE'),
      ],
      localizationsDelegates: [
        AppLocalizations.delegate,
        GlobalMaterialLocalizations.delegate,
      ],
      home: MyHomePage(title: "Flutterclutter: i18n",),
    );
  }
}

class MyHomePage extends StatelessWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Builder(
          builder: (context) {
            return Padding(
              padding: EdgeInsets.all(16),
              child: Center(
                child: Text(
                  AppLocalizations.instance.translate(
                    'string_with_interpolation',
                    {'name': 'Test'}
                  ),
                  style: TextStyle(
                    fontSize: 32
                  ),
                )
              )
            );
          }
        )
    );
  }
}

pubspec.yaml

...
dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
...
flutter:
...
  assets:
   - assets/
   - assets/translations/de.json
   - assets/translations/en.json