You'll need to download and register Gabet's
VSFilter
DirecVobSub language selection is not integrated in the player, you'll have to toggle into windowed mode, then right-click on the "DirectVobSub" green arrow that will appear in the tray bar. The last section of the menu let you access the property sheets of the filters used to render the movie.
DirectVobsub can handle internal or external, textual or bitmap subtitles, but is not integrated into BSPlayer. If you don't want to show twice some subtitles, you'll have to fine tune dvobsub in order to disable the handling of those that BSplayer can render.
...Or you can try Gabest's
MediaPlayerClassic , a very good alternative to BSplayer.