is
Determines if the SubtitleTrack should be selected by the player per default and kept in sync with the selected audio language. In that case the subtitle track will not appear in Source.availableSubtitleTracks.
Per default subtitle tracks with SubtitleTrack.isForced set to true will be selected. In case of DASH these are subtitles with the role forced_subtitle and in case of HLS subtitles with the attribute FORCED=YES.
Is called when parsing new subtitle tracks.