input: Write a function to implement a command-line interface for downloading subtitles for movies and TV shows. The function should take arguments for the release paths, language, and subtitle site, and should support commands such as "download", "exist", "test", "list", "help", and "version". If the "download" command is used, the function should attempt to download subtitles for the specified releases, print success or failure messages for each release, and return nothing. If the "exist" command is used, the function should check if subtitles exist for the specified releases and print the results. Other commands should perform their respective tasks as described in the usage section. The function should return nothing.
  assert download_subtitles(["/path/to/movie1", "/path/to/movie2"], ["enginesite1"], "en")==None
  assert is_subtitles_exist(["/path/to/movie1", "/path/to/movie2"], ["enginesite1"], "en")==None
  assert test_engines(["enginesite1"])==None
  assert "--list" command outputs the list of subtitle engines
  assert "--help" command displays the help screen
  assert "--version" command displays the version information
output: def download_subtitles(release_paths, subtitle_site, language):
      for path in release_paths:
          print(f"Downloading subtitles for {path} from {subtitle_site} in {language}...")
          # Simulate download process
          success = True  # Assume success for simulation
          if success:
              print(f"Successfully downloaded subtitles for {path}")
          else:
              print(f"Failed to download subtitles for {path}")
  
  def is_subtitles_exist(release_paths, subtitle_site, language):
      for path in release_paths:
          print(f"Checking if subtitles exist for {path} from {subtitle_site} in {language}...")
          # Simulate existence check
          exists = True  # Assume existence for simulation
          if exists:
              print(f"Subtitles exist for {path}")
          else:
              print(f"Subtitles do not exist for {path}")
  
  def test_engines(subtitle_sites):
      for site in subtitle_sites:
          print(f"Testing subtitle engine: {site}")
          # Simulate test process
          success = True  # Assume success for simulation
          if success:
              print(f"Test successful for {site}")
          else:
              print(f"Test failed for {site}")
  
  def list_engines():
      engines = ["enginesite1", "enginesite2"]  # Example engines
      for engine in engines:
          print(engine)
  
  def help_screen():
      print("Usage: download_subtitles [command] [options]")
      print("Commands:")
      print("  download  Download subtitles for specified releases")
      print("  exist     Check if subtitles exist for specified releases")
      print("  test      Test subtitle engines")
      print("  list      List available subtitle engines")
      print("  help      Display this help screen")
      print("  version   Display version information")
  
  def version_info():
      print("Subtitle Downloader v1.0")