No one has ever offered to translate one of my books, but I would decline were somebody to offer—unless I really trusted their writing ability, that is.
Translating words to another language involves a great amount of skill when art is involved, and requires a great number of judgment calls. In other words, to be a good translator of fiction, you need to be a good writer of fiction, too.
If you're planning on selling your books, also don't get a translation done unless you plan to sell that translation. When you want to sell in other languages, you need to formalize an agreement with your translator regarding their payment, their rights, etc. I suggest a flat fee, and of course be sure to obtain everything in writing via email. Consult a lawyer or your agent!
If you your book is just for fun or free, it's still good to consider what they expect for the translation, and of course, as stated previously, their ability as a writer in both English and their native language.
And well, that's about it.
No really, that's it for the book!
... or is it?