It uses it as a translation of tranne in other places, which is kind of weird. "Other than" and "except" aren't really synonyms, but certain Duolingo sentences sort of make them out to be, when translating "tranne." Point being, I still don't really get what "tranne" means. :/
Besides and except are sometimes used to mean the same thing. For example, "Except for swimming, i don't like sport" and "Besides swimming, i don't like sport". I would interpret these to mean the same thing. Maybe in the example "i eat fruit besides apples" it would make more sense with a comma, or saying "i eat all fruit, besides apples"
I know it's four months late but according to this they're interchangeable http://forum.wordreference.com/showthread.php?t=1242037
of course it is. usually the italian translation does not make a difference between both cases but in this case only "I eat fruit" really makes sense. Because the sentence probably doesn't mean "right now i'm stuffing my face with all available fruit except apples". The sentence means "generally i eat fruit, but never apples"
Translation is not something you do exactly word-for-word. We know from experience that "except for apples" means the same as "except apples", but if you take the words more literally it could mean "I eat fruit, apart from when the apples want me to. I won't eat fruit for them." This casts the apples as sentient beings and is nonsense. To make sense, the word "for" here clearly doesn't have the usual meaning of "for". So you can't assume that the usual Italian "for" word, "per", is appropriate here.