It's not a cheat. Everyone student in our school who speaks a foreign language at school is asked to do a GCSE in their language (they can refuse if they want). But it's an extra GCSE which is always a positive. The only thing is, for some uni's or colleges, or to get an English baccalaureate, they ask for a 'modern foreign language'. If you did Arabic, it wouldn't count, because it isn't foreign to you. If that makes sense? But you should still go for it.