input: Write a function to provide a command-line interface to download subtitles for movies and TV shows. The function should accept command-line arguments specifying one or more release paths, an optional language, and an optional subtitle site. The available commands are "download", "exist", "test", "list", "help", and "version". If the "download" command is provided, the function should download the subtitles for the specified release paths using the specified language and subtitle site. If the "exist" command is provided, the function should check if subtitles exist for the specified release paths using the specified language and subtitle site. If the "test" command is provided, the function should test the specified subtitle engines. If the "list" command is provided, the function should list all available subtitle engines. If the "help" command is provided, the function should display the help message. If the "version" command is provided, the function should display the version of the script.
  
  
  assert main(['download', '/path/to/movie1', '--lang=en', '--engine=opensubtitles']) == ["Downloading subtitles for /path/to/movie1 using opensubtitles in English."]
  assert main(['exist', '/path/to/movie2', '--lang=es', '--engine=subscene']) == ["Checking if subtitles exist for /path/to/movie2 using subscene in Spanish."]
  assert main(['test', 'opensubtitles', 'subscene']) == ["Testing opensubtitles engine.", "Testing subscene engine."]
  assert main(['list']) == ["Available subtitle engines: opensubtitles, subscene"]
  assert main(['help']) == ["Displaying help message."]
  assert main(['version']) == ["Subtitles Client 1.0"]
  
  
  
  assert main([]) is None
  assert main(['invalid_command']) is None
  assert main(['download']) is None
  assert main(['exist']) is None
  assert main(['test']) is None
  assert main(['list']) == ["Available subtitle engines: opensubtitles, subscene"]
  assert main(['help']) == ["Displaying help message."]
  assert main(['version']) == ["Subtitles Client 1.0"]
  
output: def main(args):
      if not args:
          return None
      
      command = args[0]
      
      if command == "download":
          if len(args) < 2:
              return None
          paths = args[1:]
          lang = "en"
          engine = "opensubtitles"
          for arg in args:
              if arg.startswith("--lang="):
                  lang = arg.split("=")[1]
              elif arg.startswith("--engine="):
                  engine = arg.split("=")[1]
          return [f"Downloading subtitles for {path} using {engine} in {lang}." for path in paths]
      
      elif command == "exist":
          if len(args) < 2:
              return None
          paths = args[1:]
          lang = "en"
          engine = "opensubtitles"
          for arg in args:
              if arg.startswith("--lang="):
                  lang = arg.split("=")[1]
              elif arg.startswith("--engine="):
                  engine = arg.split("=")[1]
          return [f"Checking if subtitles exist for {path} using {engine} in {lang}." for path in paths]
      
      elif command == "test":
          if len(args) < 2:
              return None
          engines = args[1:]
          return [f"Testing {engine} engine." for engine in engines]
      
      elif command == "list":
          return ["Available subtitle engines: opensubtitles, subscene"]
      
      elif command == "help":
          return ["Displaying help message."]
      
      elif command == "version":
          return ["Subtitles Client 1.0"]
      
      else:
          return None