input: Write a function to simulate the main functionality of a command-line interface for downloading subtitles for movies and TV shows. The function should accept an action command such as "download" or "exist", along with optional parameters like release paths, subtitle language, and subtitle site. If the action is "download", the function should attempt to download the subtitles for the specified release paths using the provided engines and language. If the action is "exist", the function should check if the subtitles exist for the specified release paths. The function should return True if the action is completed successfully, or False if there is any issue. However, if no release paths are provided, the function should return False immediately.
  assert main_functionality("download", ["/path/to/movie1", "/path/to/movie2"], engines=["opensubtitles"], lang="en") == True
  assert main_functionality("exist", ["/path/to/movie3"], engines=["addic7ed"], lang="fr") == True
  assert main_functionality("download", [], engines=["opensubtitles"], lang="en") == False
  assert main_functionality("exist", [], engines=["addic7ed"], lang="fr") == False
  assert main_functionality("unknown", ["/path/to/movie4"], engines=["subscene"], lang="es") == False
output: def main_functionality(action, release_paths=[], engines=[], lang=""):
      if not release_paths:
          return False
      
      if action == "download":
          for path in release_paths:
              # Simulate downloading subtitles
              if not download_subtitles(path, engines, lang):
                  return False
          return True
      
      elif action == "exist":
          for path in release_paths:
              # Simulate checking if subtitles exist
              if not check_subtitles_exist(path, engines, lang):
                  return False
          return True
      
      return False