I'm not sure my translation is all that incorrect... I would thing "They eat orange and apple" should be considered to be a valid translation without the indefinite article. The indefinite article is weakly implied in the gaeilge, but I would have thought the interpretation should be valid without it. Is the article strongly implied?
I wouldn't have thought "they eat orange and apple" was particularly common Engilsh. It's certainly not the way I'd expect to hear it in Australia.
Nor in the States — fruit flesh wouldn’t be spoken of in the same way that animal flesh would, e.g. “They eat chicken and beef” — but larryone’s answer could be a grammatical response to a question like “Which jelly bean flavours do the children eat?”