There's an English sentence left "Yes, I'm done" when you press quit.
Without further details on which version of Tux Paint you're using, and what language/locale you're running Tux Paint in, we can't really respond. Translators are regularly going in and bringing the translations up to date. In this case, it sounds like a Fuzzy string which needs to be un-fuzzied (the original English version of that string was altered to include a curly apostrophe, rather than the straight ASCII apostrophe, which was in there originally).
It's likely that the issue has already been addressed in CVS. If not, it will eventually. But thanks!