In Drown, Diaz launches us into a bilingual world right away. As I said before, this helps solidify the setting, and cultural background in the story. One way he achieves this is by not italicizing the Spanish words, which keeps them from popping out of the texture. In addition, he adds enough context around the word to allow the reader to infer the meaning in many cases. Lastly, he uses many of the Spanish words repeatedly, and refrains from using their English counterparts. This creates a mix between the two languages, and flows very well. It is a bit like Diaz invites us to his characters' world, to make us better understand their stories. I think Diaz uses the mix of languages for these reasons--to solidify the setting, and to help us indentify us with the characters.
In "De Daumier-Smith's Blue Period," Salinger writes the story through the point of view of a young artist, John Smith. He is definitely narcissistic, as evidenced by his painting of seventeen self portraits. He also considers himself to be high-class and sophisticated. He continually makes up stories, and some people in class called him a pathological liar, a diagnosis of which I do not disapprove. He grew up in the United States and Paris, where he started his career, and learned French. He applies to work at a correspondent school for art in Montreal, and concocts a story about knowing Picasso and being related to Honore Daumier. He accepts the position, and moves there for the summer. However, whenever he wants to distance himself from a "lower-class" person he uses French. This is well-summed up in the following scene, an altercation with a man on the bus:
"All right, buddy," he said, "let's move that ass." It was the "buddy," I think, that did it. Without even bothering to bend over a little--that is, to keep the conversation at least as private, as de bon gout, as he'd kept it--I informed him, in French, that he was a rude, stupid, overbearing imbecile, and that he'd never know how much I detested him. Then, rather elated, I stepped to the rear of the vehicle.
In this excerpt, Salinger's character speaks in French, and also denounces the people who are not in his social rank by using the French language.
In these ways, these two authors use a combination with English in very different ways. In Diaz, we are welcomed into the story, and in Salinger, the languages are meant to divide between classes.