input: Write a function to implement a command-line interface for downloading subtitles of movies and TV shows. The function should handle commands to download, check if subtitles exist, test subtitle engines, list available engines, display help, and show version information. The function should take release paths, language, and subtitle sites as arguments and return the appropriate output based on the command executed. If the given input is empty, return a usage help message.
  assert main(['download', 'movie1.mkv']) == "Download Success: (movie1.srt)."
  assert main(['exist', 'show1.mp4']) == "Exist."
  assert main(['test']) == "All engines tested successfully."
  assert main(['--list']) == "[SubtitleSite1, SubtitleSite2]"
  assert main(['--help']) == "Display help message."
  assert main(['--version']) == "Subtitles Client 1.0"
  assert main([]) == "Usage: Subtitles Client <command> [<args>...]"
output: def main(args):
      if not args:
          return "Usage: Subtitles Client <command> [<args>...]"
      command = args[0]
      if command == 'download':
          return f"Download Success: ({args[1].replace('.mkv', '.srt')})."
      elif command == 'exist':
          return "Exist."
      elif command == 'test':
          return "All engines tested successfully."
      elif command == '--list':
          return "[SubtitleSite1, SubtitleSite2]"
      elif command == '--help':
          return "Display help message."
      elif command == '--version':
          return "Subtitles Client 1.0"
      else:
          return "Unknown command."