When you are evaluating a translation service, it can be somewhat of a predicament because you may not be familiar with the target language that you need a document or website to be translated into.
You cannot assume that all translators (especially freelancers) will be dependable and provide the critically important task of translating your important messages and communicating them in localized markets.
In these cases, it's always advisable to have someone who is familiar with the target language in question and have them go over the final copies of the translated materials.
It's important to have some sort of a checks and balances if you will, where the editor of the material should not be biased on the material presented.
It's important to find a translator or translation service company that has a good reputation.
This you can do simply by referring to your immediate network of trusted associates and find out who has experienced good service with their translation needs.
Another way you can make sure you get the best possible translation results is to run a group session with a sample of your target audience if possible.
This way they can provide feedback along the lifecycle of a translation project and help provide insights not only into the quality of the direct translation, but also the degree of connection with the audience.