1) Similar to 99Designs, etc, everyone could vote on quality, so that could give you some confidence in the text.
2) Translator users could accumulate karma and reviews.
3) Translators' previous work would be public for inspection, and you could see comments and votes on previous work.
4) The requester could seek higher quality by offering a higher price.
Find the mistake in the following sentence which cost my company $X (can't tell you, but it is eyepopping):
"The subject registration screen should list the lecture subject name, drawn from the database table for the lectures subject name."
There should be an apostrophe in lecture's.
Did that cost you money?
Hitting the Big Red Button on a software project is cheaper, but not free. (For example, the Big Red Button plan for customer acceptance testing might sound like "Nobody goes home if there is an unknown risk to delivery", and a translation error anywhere means there is an unknown risk to delivery until you have verified that whatever process introduced the error was not similarly out of control elsewhere.)
Great idea.
I wonder if there are other ways to confirm data in other instances... (Where recaptha-like processing is n/a).