"Er ist zwölf, nicht?"
I accidentally wrote "Er isst zwolf, nicht?" as in "He eats twelve, no?", which doesn't make much sense unless on a specific context, yet it marked as correct. I believe I should've lost an heart there. Please correct for others.
"he is twelve, not?" this should not be a correct answer. If you had it in a sentence, like "he is twelve, not three right?", for example, it would work.
Because, ''He is twelve, not?'', isn't something you'd ever hear in English. It is in German, though.
It may be, but it doesn't make any sense to choose a "translation" in English that is not grammatically correct.
I wrote "He is twelve, isn't he?" that sounds better to me than "he is twelve, not?"and it was marked as correct.