An easy way to create such subtitles is to use ffmpeg2theora, which can create Kate streams from SubRip (.srt) format files, a simple but common text subtitles format. ffmpeg2theora 0.21 or later is needed.
At its simplest:
ffmpeg2theora -o video-with-subtitles.ogg --subtitles subtitles.srt \ video-without-subtitles.avi
Several languages may be created and tagged with their language code for easy selection in a media player:
ffmpeg2theora -o video-with-subtitles.ogg video-without-subtitles.avi \ --subtitles japanese-subtitles.srt --subtitles-language ja \ --subtitles welsh-subtitles.srt --subtitles-language cy \ --subtitles english-subtitles.srt --subtitles-language en_GB
Alternatively, kateenc (which comes with the libkate distribution) can create Kate streams from SubRip files as well. These can then be merged with a video with oggz-tools:
kateenc -t srt -c subtitles -l it -o subtitles.ogg italian-subtitles.srt oggz merge -o movie-with-subtitles.ogg movie-without-subtitles.ogg subtitles.ogg
When configuring how to rip DVD tracks, any subtitles will be detected by Thoggen, and selecting them in the GUI will cause them to be saved as Kate tracks along with the movie.