Simple app localization with JSON as input format. 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);

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

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

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

  Future<void> load() async {
    String jsonString = await rootBundle.loadString('assets/translations/${locale.languageCode}.json');

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

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

    return null;
  }

  String translate(String key) {
    return _localizedStrings[key];
  }
}

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

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

  @override
  Future<AppLocalizations> load(Locale locale) async {
    AppLocalizations localizations = new AppLocalizations(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.of(context).translate(
                    'string',
                    {'name': 'Test'}
                  ),
                  style: TextStyle(
                    fontSize: 32
                  ),
                )
              )
            );
          }
        )
    );
  }
}

pubspec.yaml

...
  # To add assets to your application, add an assets section, like this:
  assets:
   - assets/
   - assets/translations/de.json # <--- THIS LINE WAS ADDED
   - assets/translations/en.json # <--- THIS LINE WAS ADDED
  #   - images/a_dot_burr.jpeg
  #   - images/a_dot_ham.jpeg
...