If you need to have translations in your Flutter package and you can't use solutions like recommended localization delegate in MaterialApp or you can't use solutions like easy_localizations , you can try doing your own simple translations service.

Let's define translations model. It's quite simple - we're storing language code and map of translations:

class TranslationData {
  final String languageCode;
  final Map<TranslationKey, String> values;

  TranslationData({required this.languageCode, required this.values});
}

TranslationKey is simple enum with all possible translation keys from your domain:

enum TranslationKey{
  list,
  details
}

Next, you need to create your translations service:

import 'package:translations/translation_data.dart';
import 'package:translations/translation_key.dart';

class Translations {
  static final List<TranslationData> _translations = _initialise();

  static String get({
    required String languageCode,
    required TranslationKey key,
  }) {
    try {
      final data = _translations.firstWhere(
        (element) => element.languageCode == languageCode,
        orElse: () => _translations.first,
      );
      final value = data.values[key] ?? key.toString();
      return value;
    } catch (error) {
      return key.toString();
    }
  }

  static List<TranslationData> _initialise() {
    final translations = <TranslationData>[];
    translations.add(_getEnTranslations());
    translations.add(_getPlTranslations());
    return translations;
  }

  static TranslationData _getEnTranslations() {
    return TranslationData(languageCode: "en", values: {
      TranslationKey.details: "Details screen",
      TranslationKey.list: "List screen"
    });
  }

  static TranslationData _getPlTranslations() {
    return TranslationData(languageCode: "pl", values: {
      TranslationKey.details: "Ekran detaili",
      TranslationKey.list: "Ekran listy"
    });
  }
}

In this service we're initialising translations for English and Polish languages. Then in get method we're translating key for a given languageCode. If languageCode can't be found, we're using first configured language which is English.

Now, we can access our translations by calling:

Translations.get("en", TranslationKey.details);

We're still missing one thing - automatic way to get language code. We can fix that, by adding extension:

extension TranslationKeyExt on TranslationKey{
 String tr(){
   final locale = Platform.localeName;
   final languageCode = locale.split("_").first;
   return Translations.get(languageCode: languageCode,key: this);
 }
}

Now you can call TranslationKey.details.tr() to get value translated.