> the original means "Original stream's language". I thinks this is the first one. Am I right?

No it's not. It has to do with stream flags, cf. https://github.com/xbmc/xbmc/blob/master/xbmc/cores/dvdplayer/DVDPlayer.cpp#L139 and https://github.com/xbmc/xbmc/blob/master/xbmc/cores/dvdplayer/DVDPlayer.cpp#L210. But it might be reasonable to get (forced) subs for the active audio stream, I'm not sure.