From How Not to Write a Novel by Howard Mittelmark and Sandra Newman (Collins, an imprint of HarperCollinsPublishers, 2008):

It is difficult to render foreign English without falling into unintentional comedy. Some errors, however, can easily be avoided.

Do not have a foreigner address everyone as “Señor” or “Monsieur” despite speaking otherwise perfect English. Do not sprinkle the conversation of a Mexican with the few Spanish words you have picked up along the way. These are generally the same words that the Mexican would have learned in English first, like the words for “yes” and “hello.” [Original emphasis.]

It is also generally a bad idea to try to render foreign accents phonetically. An Italian saying “He’s-a gotta pretty-a daughter-a” may offend some people, but it will convince no one.

[skip skip skip]

No matter how good an ear you have, and how perfectly you’ve captured it, it soon becomes a task to read. The reader is forced to sound out each word, like somebody studying ESL, and will soon grow impatient. Instead, one or two well-placed words sprinkled throughout are enough to flavor the whole thing.

Okay, so it’s not 500 or more, but it’s still a great lesson. And a terrific book. I highly recommend it to writers of all stripes.