As to language, there is a great little article here that analyzes the use of languages (and profanity in particular). And the summation is that he's at least some people in a town like Deadwood would have sworn like that. The article does however highlight that the language has been modernized. There would have been a lot more blasphemy related profanity in a real 1890s town. And it would have been considered more serious. Today we view sex related words like cunt and fuck as the higher end of profanity... back then apparently goddamned and the like would have replaced these. We just don't take blasphemy as serious today.