[options] Clarify language tags

youtube_dl/options.py

@@ -380,7 +380,7 @@ def parseOpts(overrideArguments=None):
         '--sub-lang', '--sub-langs', '--srt-lang',
         action='callback', dest='subtitleslangs', metavar='LANGS', type='str',
         default=[], callback=_comma_separated_values_options_callback,
-        help='Languages of the subtitles to download (optional) separated by commas, use IETF language tags like \'en,pt\'')
+        help='Languages of the subtitles to download (optional) separated by commas, use --list-subs for available language tags')
 
     downloader = optparse.OptionGroup(parser, 'Download Options')
     downloader.add_option(