Ok so maybe "Bonjour, cava?" is a tad casual, but if its the only thing you can say in a language and its obvious you're just saying it to be polite, I don't think anyone is going to hassle you. True however, "Comment allez-vous?" would be better. I think people really appreciate it when you ask them how they're doing, even if they are strangers and you're a visitor.

My point is mainly to avoid starting a conversation in English if you can...they'll know youre American pretty quickly anyhow it seems.