I am one of those that you met ( I have spoke 11, 9 of them fluently at one time or another )
The problem is simple. Our map ( using the reference of 'map' from "stranger in a strange land' ) is much more confusing when communicating and some of us fight to use the correct map to convey a message.
the solution is normally ( at least for me ) is to speak slowly, and tell the other party to ask more questions. It's not that I'm being an ass, I'm trying real hard to communicate correctly.
as a side note, communicating about family and friends is easy, communicating about anger get's confusing ( try cursing someone out, and I have to think in Italian, or Portuguese, or Russian because those languages really have the ability to covey disgust ). Another set of words is 'yes' and 'no'... those words are amazingly hard since my perspective shows degrees of 'yes' and 'no'.... the most annoying word in American English for me is 'like' when used for comparison.
The one advantage I might have over those that speak only 1, is that I can re-frame an idea under another 'map' and see a slightly different perspective, bring it back to my 'map' and add that benefit.
I would hope that my little write up might help you in the future when communicating with someone that speaks multiple languages.