Translation:Kateřina knows at least ten languages.
I used "no less than" -- which, in retrospect, SHOULD have been "no fewer than" -- and that was rejected. Since "at least" and "no fewer/no less than" mean essentially the same thing, it seems like they should be acceptable translations. If I come across this again, I'll try reporting it.
UPDATE ----- Just reported my answer using "no fewer." So we'll see what happens.
UPDATE ----- A year on, I get it.
https://forum.duolingo.com/comment/25071408/I-do-not-know-whether-to-look https://forum.duolingo.com/comment/24550331/I-do-not-know-whether-to-hate-you-or-love-you https://forum.duolingo.com/comment/26095839/They-do-not-know-the-names-of-their-parents https://forum.duolingo.com/comment/26487658/Why-do-I-have-to-know-more-than-one-language