I have written and self published the book 'babymoves' in 2002. It is now in its third edition. I sell it through my website. I have been approached by a Japanese woman, wanting to translate the book into Japanese. I need to know what my rights are if I allow her to do the translation and publish it. Do I get royalties? how are these determined? etc. What other information do I need to know?
These are all details that need to be determined in some sort of contract or agreement with her. You can grant and withhold whatever you choose. Be familiar with what different rights you have over your work, so you know what's on the table.
Similarly, the terms of payment are part of your agreement. Whatever you agree to is what you get. In order to get good terms, you should get an agent or a lawyer who can negotiate your contract. You want somebody who's got the experience and knowledge to know what contract language is problematic, and who has some sense of whether the offer is "fair" in relation to other offers for purchasing translation rights in roughly the same scope.
Note that you're probably in a poor position to monitor how your work is used in a foreign language... so you might want to figure out how you can keep an eye on the translation and its use. I don't know what "typical" solutions to this are, but it's worth looking into.
All this is operating under the assumption that it's a publisher of some sort who wants to translate your book. If it's a private individual who wants to translate it for personal use... well, the same issues arise, but obviously an individual is less capable of paying you for the rights you're granting. Be sure you know what you're getting into, so you can make an informed decision.