If you have read Excalibur's guide, then you might have seen my posts regarding extraction and adding subtitle. Well, I see DVDSubEdit can now extract subtitle in SRT format (Please note that I have not fully tested it yet). You can change the texts for the language you want to add. Then convert it to sup format using SubtitleCreator, for example.
To maintain full functionalities, here are the general steps for added subtitle track:
1. First demux video, audio, subtitle tracks using PgcDemux (include celltimes.txt for chapter info). Take note of the audio delay, if any. So save the log as well for reference.
2. Get the new subtitle file ready.
3. Use MuxMan to mux all of them back along with the newly created subtitle file.
4. Use VobBlanker or DvdReMake Pro to replace the original movie VOB.
5. Use PgcEdit or DvdReMake Pro to fix the menu navigation to reference this added subtitle track, and modify the subtitle menu background with an added menu button.
Lots of work in these general steps. If you're just replacing subtitle, then in step 5 you just modify the menu background to change the button graphics to reference the new subtitle.
Tip: Test the MuxMan output first before going to step 4. If you have sync issue or subtitle display problem, you have to fix that first.