If your book is picked up by a foreign publisher, you and your translator may not be in contact at all. Try not to let that happen. Then, make sure that your translator knows as much as you do about your work's target audience, its education level, age range, reading level, interests, and so forth. Your translator should also have at least a bibliography of the material used in compiling your work. Be sure, as well, that your translator has enough time to complete the job properly.
Rush jobs reduce quality.
Finally, it helps to view your translator as a co-author of your work in the target language. Giving credit in the form of an additional byline is good practice, and if you and your translator work well together, do it again on other projects. All rights reserved.