Translation:Are you American?
In English, questions and tag-questions are interchangeable. Saying "are you American" means the same as "are you not American" means the same as "you are American, aren't you".
But strictly speaking, if there is or isn't the word "not" in the sentence, and where it is placed, does change the meaning
Oh, they all give a sort of "inflection" to the meaning: sometimes you're saying "you say you're X but I don't believe it", or "I'm pretty sure you're X ... aren't you?". But we can convey most of that using our tone of voice anyway :o)
Ultimately though, they all boil down to the same question, "Are you or are you not an American?" :o)
To which my answer is, "No."