I always assumed that the prevalence of the prison slang has to do with Russian culture's morbid obsession with prisons and crime in general. There's even quite a popular TV channel in Russia, NTV (or, НТВ), that seemingly specializes in producing crime-related series (both from the police' and criminals' perspective). Whether the Soviet imprisonments have to do anything with it, I don't know, but it's interesting to find a point of origin. I mean the kind of lingo that's not used officially but is prevalent nonetheless, mostly among the youth. I learned the word "зашквар", for example, from an acquaintence of mine who's in her early 20s, who used it without a second of hesitation or embarrasement. I sadly lack the eloquience to put it in a more correct way.What do you mean by non-mainstream language?