This is actually a big problem with how people perceive language in general. It's hard to understand if you only speak one language, but the idea of a language as a discrete unit is a myth. Here in Québec there is Québécois French, which in and of itself is a generalization, as many regions have different accents and expressions.
In France there are many kinds of French. Some people say they "can't even understand" people from a certain other region... That's pretty much where a dialect crosses the line into a language!
In general people will learn what we call "international French", which is spoken by no one but understood by all. But good luck navigating the subtleties of a French-speaking society without being familiar to their regional dialect.