docs: def mt_fields(fields, nomaster=False, onlydefaultlang=False):
      """
      Returns list of fields for multilanguage fields of model.
      Examples:
          print(mt_fields('name', 'desc'))
          ['name', 'name_en', 'name_uk', 'desc', 'desc_en', 'desc_uk']
  
          MyModel.objects.only(*mt_fields('name', 'desc', 'content'))
  
      If nomaster then master field will not be append.
      F.e.: ['name_en', 'name_uk'] -- without master 'name'.
  
      If onlydefaultlang then wiil be search only default language:
      F.e.: ['name', 'name_en'] -- without additional 'name_uk'.
  
      If nomaster and onlydefaultlang then will be use both rulses.
      F.e.: ['name_en'] -- without master 'name' and additional 'name_uk'.
      """
      assert isinstance(fields, (list, tuple))
      fl = []
      for field in fields:
          if not nomaster:
              fl.append(field)
          if onlydefaultlang:
              fl.append('{}_{}'.format(field, DEFAULT_LANGUAGE))
          else:
              for lang in AVAILABLE_LANGUAGES:
                  fl.append('{}_{}'.format(field, lang))
      return fl