If you're looking for an impartial viewpoint on how to get the best language provider, why not try the Institute of Translation and Interpreting - they're our professional body and have published a couple of useful guides for buyers, to help you find out what you need to know.
Take a look at this link for more information and to download a couple of helpful guides.
The Institute also has a Code of Conduct which we adhere to which helps guarantee that you receive a professional, accountable service from any member.