John Chu's recent post over at The Booksmugglers is a must read. He talks about the difficulty of including foreign language in works of fiction and has some truly interesting things to say on the subject.
Whereas listeners might reasonably experience that orchestration both ways, readers either understand a foreign language or they don’t. However, like how the orchestration of the Carousel Waltz must be compelling in either instrumentation, a story that makes use of dialect or foreign language must be compelling either way. Non-fluent readers must never feel as though something is missing but fluent readers must never feel as though anything is extraneous.Go on. Read the whole thing.