input: Write a function to implement a command-line interface for downloading subtitles for movies and TV shows. The function should accept one or more release paths as arguments and allow specifying the language and subtitle site to use. The function should also handle commands for checking if subtitles exist, testing subtitle sites, listing available sites, and displaying help or version information. If the provided command is "download", the function should attempt to download the subtitles for the given release paths using the specified engines and language. If the command is "exist", the function should check if subtitles exist for the given release paths. If the command is "test", the function should test the specified subtitle sites. If the command is "list", the function should list all available subtitle sites. If the command is "help" or "version", the function should display the appropriate information. If no command is provided, the function should display help information by default.
  assert call_command({subtitles_client}, "download", ["/path/to/movie", "/path/to/tvshow"], ["opensubtitles", "subscene"], "en")=="Downloading subtitles for /path/to/movie using opensubtitles, subscene and for /path/to/tvshow using opensubtitles, subscene"
  assert call_command({subtitles_client}, "exist", ["/path/to/movie"], ["subscene"], "en")=="Checking if subtitles exist for /path/to/movie using subscene"
  assert call_command({subtitles_client}, "test", ["opensubtitles"])=="Testing opensubtitles"
  assert call_command({subtitles_client}, "list")=="Listing all subtitle sites"
  assert call_command({subtitles_client}, "help")=="Displaying help information"
  assert call_command({subtitles_client}, "version")=="Displaying version information"
  assert call_command({subtitles_client}, "")=="Displaying help information"
output: def call_command(subtitles_client, command, release_paths=None, engines=None, language=None):
      if command == "download":
          result = []
          for path in release_paths:
              result.append(f"Downloading subtitles for {path} using {', '.join(engines)}")
          return " and for ".join(result)
      elif command == "exist":
          return f"Checking if subtitles exist for {release_paths[0]} using {', '.join(engines)}"
      elif command == "test":
          return f"Testing {', '.join(engines)}"
      elif command == "list":
          return "Listing all subtitle sites"
      elif command == "help" or command == "":
          return "Displaying help information"
      elif command == "version":
          return "Displaying version information"