(full disclaimer - I work at myGengo)
myGengo (http://mygengo.com) tests translators and does quality-checking for multiple levels to ensure customers are happy with the output. We also allow customers to rate translators and - I'm trying to be honest here - we're almost embarrassed to show the feedback comments because they sound too good (http://mygengo.com/talk/customers/testimonials).
Customers also have the option to request changes / corrections, and even to reject if they are not fully satisfied.
Anyways, just an alternative solution. The online-translation market gets interesting by the day :)
I've used Tomedes for translation services (http://www.tomedes.com). Also, was very satisfied with their quality. Just had a look at mygengo. I'll try it soon
That's a big concern of mine. Since I'm bootstrapping, it's going to happen that people who have no business translating will slip through the cracks. The best I can do for now is to make sure there are no repeat offenders.
You might look at these similar sites for ideas:
https://www.onehourtranslation.com/ (they also offer an interesting free tweet translation service)
http://trans-aid.jp/ (seems to be all volunteer, Japanese-oriented)