Foreign idioms can be a hazard to anyone who, like me, is trying to get to grips with another language (I keep a notebook to hand to jot any down that I find). Some translate, the French also have 'the cherry on the cake' and 'elbow grease'
but what would I do when confronted with a man with sea urchins in his pocket? Apparently not expect him to buy me a coffee - he's mean with money. Now dropping sea life into a conversation would probably alert me to a possible linguistic hurdle but to find a man 'avoir du chien' would have me looking around and reaching in my pocket for doggy treats. Apparently it means having a certain charm.
My favourite is the French 'L'heure entre chien et loup' meaning dusk. It's a favourite time of day for me as it holds a sense of anticipation edged with a slight hint of danger as the light slowly fades.
The foreign phrase I use most often is not French but Italian. I'm not given to swearing much, partly it was drummed into me as a child that it was VERY BAD but also that it is a lazy use of language. But should an oath fall from my lips then it is likely to be "Porca Miseria!"
I could list more but would love to know yours - and any useful references would be much appreciated!