I confirm. "Le" (la/les) is used 1) for a specific piece of cheese, or 2) for (Italian) cheese in general, like in "J'aime le fromage italien." (= you like all Italian cheese). If you're only pointing out that you're eating Italian cheese, it would be "Je mange du fromage italien." "Je mange le fromage italien." could be in a situation where you had to choose between several kinds of cheese and picked out the Italian one (option 1 above). Please anyone correct me if I'm wrong.
I translated this as "You are eating Italian cheese". It said it was incorrect, and should have been "You are eating the Italian cheese." But I don't understand why. Would "You are eating Italian cheese" be "Tu manges du fromage Italien", and "Le" is the same as explicitly saying "the"?
Yes, that's exactly it. "You are eating italian cheese" is "tu manges du fromage italien" while "tu manges le fromage italien" refers to a specific cheese. A possible context could be that one person bought an italian cheese, put it in the fridge and later finds someone else eating it. And now I need a snack... ;-)