Did I just lose 8 XP by making a translation?
No, I didn't. But for a while there I was fooled.
Achieved by making an edit, and then cancelling my upvote on my edit. The first time I cancelled my upvote, I got -3 XP; but the upvote reappeared, so I cancelled it again, and got -8 XP.
Reloading the page reset my XP to 2. So I didn't lose anything after all. But it's still a bug.
It was an experiment. I noticed that whenever I made an edit, I automatically voted myself up. So I wondered what would happen if I unvoted. I tried it, and the bug was quickly discovered.
Looks like you're a troublemaker! :) Probably the first in Duolingo history to downvote yourself. That's what you get for being modest.