Go for native speakers of the language. Fluency is fine for simple purposes--like finding the nearest restaurant or a taxi in a foreign country. But, when it comes to hiring a translator for business or legal purposes, you need a native speaker. This should be your first requirement for hiring them.
Perform a translation test. Simply take an excerpt from the business portfolio or legal documents that you need translated and require your prospective translator to prove they are qualified by translating it to your satisfaction.
Discuss price. You should check around online for the standard hourly rate before addressing payment with your translator. The price will very much depend on the complexity of the translation task(s).
Develop a friendly but professional relationships with your translator. If you will be working together in "live" business meetings, you will obviously need to trust the translator implicitly. If, on the other hand, the translator will simply be translating documents, you need to have open communication channels at all times to ensure the best outcome.
Create the best product possible together. You and your translator will be working as a unit, so you need to function as two halves of the same whole. Be clear about your deadlines and commitments and about your deeper concerns regarding the project at hand.
