There have been several times that I have come across in my reading dialect-type words. While they were in Paris, Brett would often use French words, or ones like "chap" and "darling" when talking to others. In this circumstance, I came across many newer words: Basque, klaxon, arriba, posada, aguardiente, and centimes. The author's purpose of incorporating dialect into his novel is to make the setting more real to the reader. It adds another aspect to his writing that most novels lack. I think that this method is effective in terms of The Sun Also Rises.
"We each had an aguardiente and paid forty centimes for the two drinks. I gave the woman fifty centimes to make a tip, and she gave me back the copper piece, thinking I had misunderstood the price." (page 112)
The only thing that would be more helpful was if he also included footnotes. I hate not being familiar with the terms he uses. Occassionally it can lead to confusion for the reader.