I have an .mp4-file, encoded from DVD with Handbrake. Now I'd like to add an extra audio track (with another language) to the .mp4 file but I haven't found a good tool to do this. Any suggestions?

I'm converting DVDs to .mp4 from unencrypted discs (we've licensed the right to do so) on a Mac. I've explored many tools for doing this (as forum regulars can probably attest..thanks guys!) and ...