I had a couple of French monks over to lunch this week (for fans of 52 Loaves, fr. Philippe from the abbey in Normandy was one of them) and although their French was quite good (fr. Lu has been in the States for two years) the conversation was continually interrupted by having to explain English idioms. You have no idea how many idioms we use without even thinking about until you speak with a foreigner.
We tend to think of idioms -- phrases whose meanings cannot be deciphered by the meaning of the component parts -- as colorful expressions like "kick the bucket" or "pie in the sky" but in fact they're far more common (really, I'm not pulling your leg), and it's hard to speak more than a few sentences without using one of them and inadvertently throwing your guest for a loop (see?).
My favorite French idiom did not, thankfully, come up at lunch, for I restrained myself from saying, after taking a sip of the fine red wine I'd opened for the occasion, "C’est le petit Jésus en culotte de velours!" -- It's baby Jesus in velvet shorts! This is the French version of "It's the cat's pajamas!"
I suspect that wouldn't have gone over well with my monks. But we had a marvelous time -- I got to speak a little French, we all got to eat, and we had a swell (not the ocean kind of swell) time.