Wasn't it Solzhenitsyn who said something about the prevalence of prison slang in the Russian language after the increase in prison sentences starting in the 20s? I'll try and find the actual quote once I get home, but if I remember correctly, he mentioned something like that in The Gulag Archipelago (or was it The First Circle?). In any case, the prison slang is there and it doesn't seem to be going away. What do you mean by non-mainstream language?
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?