"Kateřina is our youngest player."
Translation:Kateřina je naše nejmladší hráčka.
Czech has different words for the men and women who are in the same role, for example, přítel/přitelkyně (friend), kamarád/kamarádka (also friend), doktor/doktorka (doctor), herec/herečka (actor), majitel/majitelka (owner), šef/šefka (boss), etc.... and, as we see here, hráč/hráčka (player).
Since we know the person in this sentence is (highly likely to be) female, I would expect the female noun to be used. If the native Czech speakers on the team feel, based on your suggestion, that the "male" version is acceptable, it will be added to the list of valid translations.