input: 
  from os import path
  from sys import argv
  from docopt import docopt
  
  from engines.engine import SubtitleSite, SUBTITLE_SITE_LIST, DEFAULTS
  
  __doc__ = \
  """
  Subtitles Client
  
  Usage:
    {prog} download <releases_path>... [--lang=<language> --engine=<subtitle_site>...]
    {prog} exist <releases_path>... [--lang=<language> --engine=<subtitle_site>...]
    {prog} test [<engines>...]
    {prog} (-l | --list)
    {prog} (-h | --help)
    {prog} (-v | --version)
  
  Options:
    -l --list                 Show subtitles engine list.
    -h --help                 Show this screen.
    -v --version              Show version.
    --lang=<language>         Subtitle language (alpha2) [default: {def_language}].
    --engine=<subtitle_site>  Subtitle site              [default: {def_engine}].
  """.format(prog=path.basename(argv[0]),
             def_language=DEFAULTS['subtitle_language'],
             def_engine=DEFAULTS['subtitle_engine'])
  
  def main():
      args = docopt(__doc__, help=True, version='Subtitles Client %s' % __version__)
  
      if args['download']:
          download_subtitles(args['<releases_path>'], args['--engine'], args['--lang'])
      elif args['exist']:
          is_subtitles_exist(args['<releases_path>'], args['--engine'], args['--lang'])
      elif args['test']:
          test_engines(args['<engines>'])
      elif args['--list']:
          for sub_site in SUBTITLE_SITE_LIST.keys():
              sub_dict = SUBTITLE_SITE_LIST.get(sub_site)
              print sub_dict.get('class_name')
  
  if __name__ == "__main__":
      main()
  
  **What is the primary purpose of the `is_subtitles_exist` function in this script, and how does it handle multiple subtitle engines specified by the user?**
output:     
  def is_subtitles_exist(releases_path, engines, language):
      results = {}
      for release_path in releases_path:
          results[release_path] = {}
          for engine in engines:
              sub_site = SUBTITLE_SITE_LIST.get(engine)
              if sub_site:
                  site_instance = sub_site.get('class_name')()
                  results[release_path][engine] = site_instance.check_subtitle_exist(release_path, language)
              else:
                  results[release_path][engine] = False
      return results
  