langauge.dart

abstract class Language {
  factory Language(String languageCode) {
    if (languageCode == 'ko') {
      return Korean();
    }

    return English();
  }

  void sayHello();
}

class Korean implements Language {
  @override
  void sayHello() {
    print('안녕하세요');
  }
}

class English implements Language {
  @override
  void sayHello() {
    print('Hello');
  }
}

void main() {
  Language('ko').sayHello(); // 안녕하세요
  Language('en').sayHello(); // Hello 
}