If the subtitles have been burned into the the movie, you cannot turn them off.
Most dvd players have a subtitle on/off option
To add subtitles to a foreign file you need to download the appropriate SRT file.
You will find them here ....
http://subscene.com/ or
http://www.allsubs.org/Name the srt file EXACTLY the same as the movie and put them both into the same folder. Play with VCL (Free from videolan.org)
The SRT file is small and easily edited
You can then burn the subtitles with Xtodvd