There may be other ways, but Overdubbing springs to mind immediately.
Overdubbing process was designed so you could play an existing drum track (for example) and sing or play multiple parts to it. First guitar, then voice, then violin.....
Normally, each part arrives on its own track divorced from the others except for time. This allows you to tune or adjust each instrument or voice as needed before smashing everything together into a final song.
You would have the original performance on the first track and translation on the next. I understand Spanish and English don't work out that way (a friend is a court translator), but that may be the easiest way.
Yes, each time you press Record, Audacity will start a whole new track, but only in overdubbing do you hear the original or older tracks while you're doing it.http://manual.audacityteam.org/man/tuto ... rdubs.html