I think it's best not to attempt it unless you're sure you can get it right. Speak to someone who is fluent beforehand, ensure you're getting it right, rather than making a fool of yourself. Done correctly, I'm sure it makes a good impression, because you'll have put the effort in. I'd stick to the basics... just a greeting, a farewell, and how to say please and thanks.
As for avoiding them assuming you are fluent, try to follow up with with something in English before they get a word in edgeways. Make it clear you're making a token effort, not learning the whole language just for them.