When is it OK to use slang in French?

Here is her question in its entirety:

Well, it’s all a question of social class and also what kind of slang is being used.
In my opinion French slang carries a negative stigma only in the more bourgeois layers of the population, not in the middle and lower classes (not even among the “nouveaux riches”).
Now, there’s no one French slang, but many. I even can safely say that French slang is richer than American English slang. Once again, we’re talking about slang and not curse words and I assume that the situation is far more different (thus more similar to France) with British English.

And both things (the fact that it has a negative stigma among the bourgeois and the fact that it’s very rich) are totally linked.

Most French slangs come from working class jobs. Many different professions used to use specific terms -related or not to their profession- to communicate. Partly because back in the days people tended to stay among their own much more than today. Of course, the rich didn’t hang out with the poor (neither today), but also, let’s say butchers tended to hang out with other butchers, etc. Competition was not as much of an issue back then (think about guilds and such), and they all ran into each other constantly, when going to Les Halles for example and in Paris’ case.

Concerning Les Halles, you need to know that before being a crappy shopping mall, it was the main (and only?) wholesale market of Paris where all the products coming from farms all over France where send to then sold to Parisian retailers. I’m mentioning this here, because it has its importance as Les Halles is more or less the birthplace of most Parisian slangs.

So, people from the same profession tended to develop similar words and expressions, partly because when you are a more or less tight group you tend to develop your own expressions and ways of expressing things (this is roughly why there are many languages in the world and not just one), partly because it could be used as a “secret” language when they wanted to talk in front of a costumer or somebody else without wanting to be understood by said person.

On the other hand, the bourgeoisie and the upper class in general (it was the same thing until quite recently) also tended to develop their own words and expressions, but as they were the dominant class, it didn’t become slang, but new words and expressions in the official language.

Which brings us to today where professional slangs have partly disappeared, partly merged into what we call today “French slang”, but to the eyes of the bourgeoisie nothing has changed, using slang still has this lower class connotation, hence the fact that they think poorly of it.

And if this is a very Parisian phenomenon, one must also mention regional slangs, which are words and expressions developed in specific regions of France and which carry the same stigma because of the fact that the dominant class considers non-Parisian France pretty much the same way it considers the lower Parisian classes, with much disdain.

But while I’m sure you’re thankful for all of this information and background, you might also think that doesn’t really help you in your everyday life.

So what’s the situation today with the use of slang?
When is it OK to use slang, and when is it not?

My answer is simple (and at the same time, not): It depends who you’re talking to.
The problem for non native speakers of French is that it’s a very intuitive thing (like it is with every other language, it’s not specific to French). It’s growing up with the language that makes you know when it is OK to use a certain word or expression and when it’s not. Especially because slang is not a unified thing. There are many slangs, and even many different levels of slangs in “one” slang.
So if you’re not a native speaker of French, the only thing I can tell you is that to be adventurous and try using certain words with different people and see their reactions to it. And if you’re not adventurous, just listen to people from different social backgrounds and pay attention to the words they use and the words they don’t.

Also keep in mind that this feeling of the language (as opposed to the knowledge of the language) is pretty much the last thing one gets when learning a language. Once you’re there, you’re completely bilingual. And I know very few people that are there with French. I even remember my professors in the US who spoke perfect French (perfect grammar of course, but also perfect pronunciation, even their accent was sort of French) but once in a while they would misuse a familiar term and that would betray them…