The thing is, though, if the right idiom is used in the right sentence, it sounds and feels good; but as with all good things, too many idioms will make you look like you’re trying too hard to impress your audience.
The same caution applies to slang in general. Yes, French people do use a lot of slang: “merde” (shit) is common, so is “con/conne (dumbass, asshole/bitch), “putain” (lit. whore but used for so much more – see video below), and other less-than-charming words. But what sounds perfectly normal from the lips of a French person, sounds contrived coming from a foreigner.
Furthermore, slang is used in a certain context; if you use it with the wrong people, or in the wrong situation, it will be a big faux pas.
I remember this time when I was having dinner in Boston with a student of mine and his very proper and old-Boston like wife. I was… 25? They were in their 70ies. I allegedly drank a bit too much wine at dinner, but the wine was good, and I was a bit impressed with the invitation, so I thought wine may help me ease up a bit… and it did. A bit too much. I was trying to tell a story, and put emotions in it, and at a loss for words I said “it was such a shitty experience”…. The wife said “Of my!” and I realized I was out of place.
So, choose your “slang” words carefully. And never, ever translate a slang expression in another language. What is no big deal in your language may be very offensive in another language. The literal translation of “con” in French is a 4 letter word starting with a c in English. But the meaning has evolved through time, most French people won’t know the original meaning and we nowadays use “con/conne” all the time; I’ve known better than to use the English literal translation…
If you like slang however, check out Olivier’s series of post about “Le Parler D’jeunes“: the slang of the young French generation. Slang evolves very fast, and you don’t want to be using out-dated slang expressions (nor should you use too hip sounding ones after a certain age…)
My advise: learn slang so you can understand it. But use is sporadically, and only when you are sure the setting and timing are appropriate.
Here is a fun video to illustrate the many uses of the word “putain”. Although quite vulgar, I have to admit we all use it a lot – well, maybe not my Mom…