In books, movies or comics author look for:
1. Do a really good Spanish phrase, accurate to the year (30s, 40s, 50s, 90s, etc.) and to the country of origin of the speaker.
2. But some just like to impress their English speaking readers, for this case they use correct grammar and wording but they use a couple of very well known Spanish word for the English speaker to reckon and gratify on their knowledge of Spanish.
If you tell me what is the style you want to give to the tranlation I can help you with it send me a private e-mail to smabres at yahoo dot com.