Recently, I was having a conversation at work and astonished myself by hearing the words une nana come out of my mouth. Nana is French slang for 'chick', and I've undoubtedly picked it up (after always regarding it as a faintly ridiculous thing to say, given that its connotations in English are roughly diametrically opposite to in French) from my manager, who invariably refers to women he's come into contact with as "cette nana (qui travaille avec Roméo, qui m'a envoyé un mail hier, etc.)" I don't think it's meant to be offensive, I think he uses it just as the female equivalent of "ce mec" (this guy), it's just that, in French as in English, slang terms for girls carry baggage whereas those for guys generally do not.
That got me thinking about the perils of using slang in a foreign language. The next time I refer to someone as une nana (kind of hope there's not a next time, but hey) am I going to wind up inadvertently offending someone? Or will I slip up and follow my female boss's example of saying ça me fait chier! (which means 'that pisses me off', but literally 'that makes me shit') in a meeting? Even if I'm using the slang correctly, does it come off as being weird somehow? We've probably all encountered a non-native speaker of English who secretly makes you giggle when they bust out with "'sup dawg" when most of the time they can barely string a coherent sentence together. How does one find a balance between not sounding like someone's nana (in the English sense) on the one hand, and not sounding like a silly white girl who's watched La Haine (a famously slangy - and very good - French film set in the 'ghettos' of Paris) too many times? How do you judge whether a word that seemed fine when talking to your friends is okay to use with your neighbour? I was horrified when I learned that dégueulasse, which I'd just thought was along the lines of 'gross', could be considered closer to 'effing disgusting'. And it might be almost as bad if you drop a "slang" term in that was last current c. 1974 (like if someone told you something was "groovy, hepcat, can you dig it?")
I know, more from teaching English than from learning French, that there are different schools of thought on whether learners should be taught slang and other non-standard forms. The first camp holds that it will just confuse learners and lead them to making mistakes and embarrassing themselves, so you shouldn't teach 'bad' English/French/etc. and you should just tell students it's wrong if they ask you about it. I can understand not confusing someone who's just learning how to say "Comment-allez vous?" with all the different other ways to say this (the old dude at work who apparently speaks 'country French' - I often fail to understand him - confused me once by asking me "tu vas?" which literally means "are you going?", without the normal "how..." in front of it. I answered "Where?"...) but I think at a minimum you should acknowledge that not everyone speaks like your French textbook.
For one thing, language can change in more than one 'direction', so to speak. Words that were once acceptable or common may not be a couple of decades later. I've heard learners of English talking about "feeling queer", for example, and I remember the textbook we taught from in our CELTA course encouraging students to talk about their "turn-ons". In a French context, I've heard of many people, who learnt French in different countries, being tripped up by their textbooks or teachers telling them that baiser (the verb) means 'to kiss', whereas that meaning has been thoroughly supplanted by 'to f-ck'. Mysteriously, almost everyone seems to learn that the hard way!
When you get to a more advanced level in a foreign language, I think you should be actively providing students with examples of slang and other non-standard usages. It should be clear that it IS slang, but even if you tell them not to try using it themselves, it's always useful to actually be able to understand people (once you've got over the hurdle of being able to understand basic French, obviously). It seems to be a common sentiment (I certainly think so, although I don't know whether or not it's objectively true) that the French are pretty bad at slowing down and dumbing down their speech for foreigners, so you can't necessarily expect that a conversation won't be sprinkled with exotic argot.
The final phase is using slang yourself. I have some terrible habits in French, such as dropping the 'ne' in negation every time (everyone does this, but a certain type of French person will scold you for it because you're a foreigner) and ending sentences with quoi? waaay too often (kind of like 'innit?' in a UK-English context, or 'au?' or 'eh?' in NZ-English. Americans and others feel free to suggest what the US etc.-English equivalent may be), but most of the time I feel I use slang more or less appropriately, even if I sometimes feel secretly a bit foolish talking about bagnoles and fringues (cars and clothes), etc.
What about you guys? Do you feel you've mastered French slang? Or have you ever used it when you shouldn't have? Do you think foreign speakers seem silly using slang? Or sillier if they are talking like they've swallowed a 1950s phrasebook?