I always try to learn and use at least politeness phrases in the local language. That shows respect. If my interlocutor thinks I am fluent, I quickly smile and say "no".

You know the old joke: What do you call someone who speaks several languages?

A. A polyglot.

What do you call someone who speaks two languages fluently?

A. A bilingual.

What do you call someone who speaks only one language?

A. An American.