Just by logic. If I want to ask you for your nationality I will say: Are you brazilian? Você é brasileiro? Eres brasileño?
Because I am asking you, you are not unknown to me. I will never say: Are you a brazilian? Você é um brasileiro? Eres un brasileño?
Do you see how ridiculous sound it in any language?
No, it does not sound ridiculous to me. At the most unusual, but not grammatically incorrect. Actually the intention of my question was more generic, not necessarily in this context. It seems that Arab has no article, so it seems to be impossible to make this distinction.
You could try this: The articles "a" or "an" are unnecessary in nationalities if I am speaking directly to the listener. Like in this example, someone is talking with Reem.