input: Write a python function to check if subtitles exist for a given release.
  assert is_subtitles_exist(["movie1.mkv"], engines=["site1"], lang="en") == "Exist"
  assert is_subtitles_exist(["movie2.mkv"], engines=["site2"], lang="fr") == "Does Not Exist"
  assert is_subtitles_exist(["movie3.mkv"], engines=["site3"], lang="es") == "Exist"
output: def is_subtitles_exist(releases, engines, lang):
      # Mock data for the sake of example
      subtitle_database = {
          "site1": {"movie1.mkv": ["en", "es"], "movie2.mkv": ["fr"]},
          "site2": {"movie2.mkv": ["en", "es"], "movie3.mkv": ["fr"]},
          "site3": {"movie3.mkv": ["en", "es"]}
      }
      
      for release in releases:
          for engine in engines:
              if engine in subtitle_database and release in subtitle_database[engine]:
                  if lang in subtitle_database[engine][release]:
                      return "Exist"
      
      return "Does Not Exist"