If dvdstyler does not for some reason recognize subtitles in a video stream it is given, there is no way to specify a language and the dvdauthor will create the DVD without a subtitle language specified (becomes "??" with many players). It would be nice if eg. the default subtitle language were given to dvdauthor even if dvdstyler had not found subtitles.
This has been a problem with dvdstyler as long as I remember, currently I'm using the pre-packaged dvdstyler-2.3.5-2.fc17.x86_64.rpm