Reading Nabokov's Luzhin's Defense, I encountered the following sentence:
"Oh, Luzhin," she sighed, "what you do to the language."
Reading the (quite competent, as far as I can judge) English translation by Michael Scammell, I did not notice much amiss in Luzhin's language. His vocabulary does seem limited, and his syntax simple, but I don't remember genuine mistakes or coarseness. I am guessing that in the Russian original, Luzhin's speech is more markedly uneducated. If you've read it in Russian, I'd love to hear what you think about this.