Do you speak English?
What do you call someone who speaks two languages? Bilingual. What do you call someone who speaks only one language? An American. The question is pretty much how Americans traveling abroad greet the locals.
Saying "Do you speak English?" can either get you around or get you kidnapped, tortured, killed and having your family discover your fate on CNN anywhere around the world. Asking the question is ok in a place where you see a McDonald's. It is not ok and you should avoid places where electricity and potable water are a luxury to ask the question.