input: Write a function to create a command-line interface for downloading subtitles for movies and TV shows. The function should accept release paths as arguments and allow specifying the language and subtitle site. The function should handle commands such as "download", "exist", "test", "list", "help", and "version".
  assert download_subtitles(['/path/to/movie'], ['opensubtitles'], 'en')==None
  assert is_subtitles_exist(['/path/to/tvshow'], ['subscene'], 'es')==None
  assert test_engines(['opensubtitles', 'subscene'])==None
  assert main()==None
output: def download_subtitles(release_paths, sites, language):
      pass
  
  def is_subtitles_exist(release_paths, sites, language):
      pass
  
  def test_engines(sites):
      pass
  
  def main():
      pass