No place to report, but the English sentence is unnatural. We would say, "Which food did you eat?" or "What kind of food did you eat?"
Though not correct in British English, in Kenyan English a sentence like "You ate which food?" is common.
I would say that it is not so much unnatural as a) possessing different connotations as to the subtleties of what is being asked, and b) substandard grammatically if one truly wishes to be pedantic.
You can report it in the phone app (at least in the Android version). There's a free text field.
"You ate which food?" would be natural in American English if the emphasis was on "which," expressing the speakers already heard which food was eaten but is surprised by what the other person ate. Haha, does that make sense?