While in present tense you can't see the gender of the subject, in past tense and future compound - you can. So here it will be:
He -> (On) jadł.
She -> (Ona) jadła.
It -> (Ono) jadło.
Similarly, if you were speaking directly to someone, it would be "(Ty) jadłeś" when you speak to a man, and "(Ty) jadłaś" when you speak to a woman. That applies to every person, including "I".
I believe that the majority of Polish learners of English would translate it to "supper". I would.
But the 'meals of the day' distinction has proved to be one of the most problematic things in this course. It's a cultural thing between different parts of the English-speaking world. You have (simplifying) the 'American way', which translates śniadanie/obiad/kolacja to breakfast/lunch/dinner. And then you have the 'British way', which translates those to breakfast/dinner/supper.
We decided to always use the American version as the default (while accepting the British one, of course) as Duolingo is an American company. And it turned out to be a good decision, because the amount of complaining/doubts/questions has decreased significantly. But you can always translate "kolacja" to "supper" if that's a more natural answer to you.