Do not compromise on this. No matter how schooled we may be, none of us can handle the subtleties of a second language the way we can our mother tongue. Also, make sure your translator has not lost touch with the target language through years of living abroad. He was astounded at how much the language had changed over the years when he made a visit to his home village in the early '90's. Merely knowing two languages, however, does not make a translator.
Good translators are also good writers.
Have prospects translate a small sample of your work as an audition. Then have a native speaker of your target language, not a translator, read the translation and tell you how it scans. In today's world, it can also be important to hire a translator who is computer-literate. Look for the membership lists on these sites. Narrow your search further by focusing on literary translators.