input: Write a function to retrieve all unique language tags from the response class properties.
  assert set(get_all_languages(response_instance)) == set(['nl', 'en', 'fr'])
  assert set(get_all_languages(empty_response_instance)) == set([])
output: def get_all_languages(response_instance):
      languages = set()
      for prop in response_instance.class_properties:
          if hasattr(prop, 'language'):
              languages.add(prop.language)
      return list(languages)