Yes, it definitely makes sense in Italian, and I would argue, despite not being a native English speaker, that you can say "we eat a sandwich". Am I wrong? Actually, this issue has arisen in the German forum, and other people are under the same impression:
http://duolingo.com/#/comment/89596 Lenkvist - I don't really understand what the fuss is about. In English people can say "we are going to grab a coffee" and it is still clear they do not mean one coffee for the whole group.
While the example about coffee is correct (in that it's commonly used), and it would be were you talking about sandwiches as well, I do not know of anywhere where the actual act of eating or drinking is described in this way in English. I'm neither a linguist nor an etymologist, but I would say as a native (American) English speaker, that it is not correct to refer to a singular sandwich in this case unless you mean to communicate that one sandwich is being shared.
Of course, that doesn't make the sentence incorrect, just a bit odd.
Well you found the solution yourself. One sandwich is being shared. You will often find odd sentences here-and sometimes they are just odd-but other times there might be a bit of method to the madness. This sentence for example, we all expected sandwiches. Some of us lost hearts because we didn't notice it was singular.
I sometimes refer to a single sandwich when I mean two. E.g. 'we'll grab a sandwich on the way there', or 'we had a sandwich earlier'. I'm sure this is fine, whether or not linguists would agree.
Having said that, the translation 'we do not eat a sandwich at the zoo' is odd. 'We ate/got/had/bought...'; 'we'll eat/get/have/buy....'; 'we didn't eat/get/have/buy...' would all sound ok.