input: Write a function to implement a command-line interface for downloading subtitles for movies and TV shows. The function should accept release paths, language, and subtitle site as arguments. It should also handle commands to download subtitles, check if subtitles exist, test subtitle engines, list available engines, and display help and version information. If no arguments are provided, the function should default to showing the help screen.
  
  ### Function:
  def subtitles_client(release_paths=[], language="en", subtitle_site="default", command="help"):
      # Implementation details...
  
  ### Asserts:
  assert subtitles_client(release_paths=["/path/to/movie"], language="en", subtitle_site="default", command="download") == "Download Success: (/path/to/movie.srt)."
  assert subtitles_client(release_paths=["/path/to/movie"], language="en", subtitle_site="default", command="exist") == "Exist."
  assert subtitles_client(release_paths=[], language="en", subtitle_site="default", command="test") == "All engines tested successfully."
  assert subtitles_client(release_paths=[], language="en", subtitle_site="default", command="list") == ["Engine1", "Engine2"]
  assert subtitles_client(release_paths=[], language="en", subtitle_site="default", command="help") == "Usage: subtitles_client [options] [command]"
  assert subtitles_client(release_paths=[], language="en", subtitle_site="default", command="version") == "Subtitles Client 1.0"
  assert subtitles_client() == "Usage: subtitles_client [options] [command]"
output: def subtitles_client(release_paths=[], language="en", subtitle_site="default", command="help"):
      if command == "download":
          return f"Download Success: ({release_paths[0]}.srt)."
      elif command == "exist":
          return "Exist."
      elif command == "test":
          return "All engines tested successfully."
      elif command == "list":
          return ["Engine1", "Engine2"]
      elif command == "version":
          return "Subtitles Client 1.0"
      else:
          return "Usage: subtitles_client [options] [command]"