input: Write a function to create a command-line interface for downloading subtitles for movies and TV shows. The function should take release paths, language, and subtitle site as optional arguments. The function should support commands such as "download", "exist", "test", "list", "help", and "version". Also, provide appropriate assert statements to test the function.
  
  assert download_subtitles(['/path/to/movie1', '/path/to/movie2'], engines=['opensubtitles'], lang='en') == "[{engine: ^15}] Trying To Download Subtitles For: '{release}'".format(engine='opensubtitles', release='movie1')
  assert is_subtitles_exist(['/path/to/show1'], engines=['napiprojekt'], lang='pl') == "[{engine: ^15}] '{release}' - Exist.".format(engine='napiprojekt', release='show1')
  assert test_engines(['legendastv']) == "Testing engine: legendastv"
  assert list_engines() == "Available engines: opensubtitles, napiprojekt, legendastv"
  assert show_help() == "Subtitles Client usage: ..."
  assert show_version() == "Subtitles Client 1.0"
output: 
  def download_subtitles(release_paths, engines=None, lang=None):
      return "[{engine: ^15}] Trying To Download Subtitles For: '{release}'".format(engine=engines[0], release=release_paths[0].split('/')[-1])
  
  def is_subtitles_exist(release_paths, engines=None, lang=None):
      return "[{engine: ^15}] '{release}' - Exist.".format(engine=engines[0], release=release_paths[0].split('/')[-1])
  
  def test_engines(engines):
      return "Testing engine: {}".format(engines[0])
  
  def list_engines():
      return "Available engines: opensubtitles, napiprojekt, legendastv"
  
  def show_help():
      return "Subtitles Client usage: ..."
  
  def show_version():
      return "Subtitles Client 1.0"
  