You can do this using Customize or Main Movie mode in DVDFab DVD Copy or using the DVD Ripper module to burn the subtitles into the video when you make a MP4/AVI/MKV etc. file conversion.
For the DVD Copy modes, select the subtitles you want by putting a check in the box for the correct stream, then click just to the left of the box so that a small arrow or triangle appears. These subtitles will now play automatically whenever you play the DVD with no intervention necessary. You can turn them off using the remote if desired.