"What do you eat?" is a very general question. If you asked someone "What do you eat?" and they said "Kissel." that would kind of imply that they only ever eat kissel. In the context here, it's pretty clear that this is a question that someone asks someone in the middle of eating. You would have to say "What are you eating?"
Using the simple present tense in English usually implies a habitual action, or statement of facts. It would make sense if someone was asking about your habits, for example "What do you eat for breakfast in the mornings?" As AGreatUserName said, it's more likely that we'd say that when we saw someone eating something, and therefore use the present progressive tense.
That said, without context we cannot really declare that it's "wrong," so go ahead and report the missing translation, as long as you're sure you haven't made a mistake elsewhere in the sentence.
My Finnish cookbook, written in English but with the names of the dishes in both languages, has recipes for both fruit soup - hedelmäkeito - and fruit kissels - marjakiisseli, etc. The directons are basically the same. And none of them include oats, which is what I found on the internet. Dunno what it all means. Regional, I guess.