Many languages have different varieties of slang - often used by a small percentage of the population (e.g. teenagers from working class suburbs) with the deliberate intention of being incomprehensible to people outside the group.
French, though, has an extensive slang vocabulary which almost everybody understands even if they don't use it. A sentence such as
"La frangine du toubib a flingué son clébard et fait cramer sa bagnole." is understandable to a very large percentage of the population but probably to rather few foreign students of French.
(it means "The sister of the doctor shot his dog and set fire to his car").
The use of this slang marks the speaker as being casual, very informal and unimpressed by the audience.
I can't think of anything equivalent in English; the effect is something like saying "The bloody sister of the f*cking doctor bloody well shot his f*cking dog...""
I asked some German friends and they also said that they don't know of anything similar in German.
Does anyone have examples of something similar in other languages?