Am I right in assuming one can only add one subtitle track to the movie?

If so, it looks like a tool to sync titles with everything else in place and then export it into a srt file and repeat steps with other subtitle streams. In the end you then need a proper tool to author DVDs or AVCHD or BluRay discs to include all those subtitles. Of course the entire subtitling can be done using another tool - open source Jubler comes to mind. But I do like an "All-in-1" tool to do all video, audio and subtitle editing if possible. And a working disc producing part allowing to smart-fit the disctype selected.

Apart from (ex) Ulead's DVD Workshop V2 and AVCHDCoder I am not aware of any sofar.