A Cornish miner is depicted as needing an interpreter. The last monoglot Cornish speaker died in the 18th century. The language was extinct at the time Deadwood was founded.
The Cornish are actually speaking Irish.
Sol is seen reading the Sears Roebuck catalog in 1877. The Sears Roebuck Company was founded in 1892. (Richard Sears founded a mail order watch business in 1886.) The catalog Sol reads appears to be a replica of the 1892 catalog.