Language is dependent upon the social class, education, location, and vocation. Good rule of thumb is to just go with language we use today - but leave off the slang, and make sure that you aren't using words that came into the vocabulary after the time frame of your story.
One of the great mistakes people make when writing about the west is assuming they were different from the way we are now. When dealing with the west, esp. post Civil War, you must take into account immigrants, are they from the North, South, Midwest. The Earps were basically from the Midwest. There would be an Iowa inflection in the way they spoke. John Wayne copied the way Wyatt Earp talked. You want to know the way Wyatt talked, how he sounded, the language he used, just watch a classic John Wayne western. One of the great mistakes made with Doc is that they use a Tidewater inflection in his accent. His family was from northeastern Georgia, near Oconee County, SC. If you know an old timer from Anderson, Oconee, Pickens, in SC, and Hart, Elbert in GA, you can figure out how Doc spoke. The McLaurys were from upstate New York. The Earps, Doc, and McClaurys were fairly well educated men who would not go around sounding like they do in "classic" westerns written around the turn of the century.
You need to understand that the "classic" writers used the vocabulary of their location, vocation, etc. Having a lawman in the vein of Wyatt, Bat, Luke Short, etc. sound like an uneducated country bumpkin is just plain wrong. Over half the cowboys were former slaves. A goodly number of cowboys were former Confederates, which must have made for some tense times. By the late 1870s most large ranches preferred hiring former Buffalo Soldiers, most of whom came from the South.
Very few people who lived in the "wild west" were from there. The exception would be in the southwest, where the Hispanic population would be speaking Spanish as a primary language, and might not even speak English.
Don't fall into the trap of assuming that novelist know what they are doing, even the classics. Louis L'Amour is the exception. Unless a person in the west was illiterate, you will find they were as well read as possible. There was very little to do, except for reading. Families would have a King James Version of the Bible. Many would have Shakespeare. Quite often, that's how people learned to read. That would reflect their vocabulary.
Wyatt was extremely well educated (self educated), probably the rule, not the exception. He was a voracious reader. He kept books in his saddlebags, even when out riding. He would go to the local library where he lived and check out stacks of books, devouring them. His favorite western was the Virginian, which he said 'got it right'. He read and re-read the book.
What truly annoys me is to read something that is an insult to the intelligence to the reader, and the time period. They were not ignorant wretches. Their language skills, on average, were far superior to ours today. Their vocabulary was superior to ours. Their slang was different, more interesting.
SJR
SJR