A simple but very usefull feature (specially when you are not american) could be that the mass cd adder function try to find the subtitles file on the cd, plus the movie file of course. This can help to filter the movies for the ones with subtitles and the ones without.
The only thing to add is a list of checkbox with the possible extensions for the subtitles files, and make the program to find this files and add to the "subtitle" field (allready present)