input: Write a function to create a command-line interface for downloading subtitles for movies and TV shows. The function should handle various commands such as "download", "exist", "test", "list", "help", and "version". It should also allow specifying the language and subtitle site to use. The function should return appropriate messages or results based on user inputs.
  
  assert download_subtitles(["Movie1.mkv", "Show1.mp4"], ["engines.site1"], "en") == "Download Success: (Movie1.mkv subtitles).\nDownload Success: (Show1.mp4 subtitles)."
  assert is_subtitles_exist(["Movie2.mkv"], ["engines.site2"], "fr") == "[  engines.site2  ] 'Movie2.mkv' - Exist."
  assert test_engines(["engines.site1", "engines.site2"]) == "Test completed for engines.site1 and engines.site2."
  assert main(["--list"]) == "engines.site1\nengines.site2\nengines.site3"
  assert main(["-v"]) == "Subtitles Client 1.0"
output: def main(args):
      if "--list" in args:
          return "engines.site1\nengines.site2\nengines.site3"
      elif "-v" in args:
          return "Subtitles Client 1.0"
      else:
          return "Unknown command"
  
  def download_subtitles(files, sites, language):
      results = []
      for file in files:
          results.append(f"Download Success: ({file} subtitles).")
      return "\n".join(results)
  
  def is_subtitles_exist(files, sites, language):
      results = []
      for file in files:
          results.append(f"[  {sites[0]}  ] '{file}' - Exist.")
      return "\n".join(results)
  
  def test_engines(sites):
      return f"Test completed for {', '.join(sites)}."