How do you ensure that the quality of the translated files you asked for is great?

After the translator finishes his work comes the job of the "proofreader", and that's his job! without him, and if you do not speak the language you should at least review it by one person who does. Or simply wait for bad reviews (because no one will applause for a good one).

Yes

At first you must have a professional approved documents then you can make crossing between the translated files according to spesefication and quality control plan. 

When you feel the customer read and understood it easily and is satisfied. When you translate hum/her as per his/her understanding.

There is always a criteria for recruiting someone. The quality of his work/deliverables depends upon how good the criteria and evaluation had been conducted before appointing him. Regarding cross checking his quality of work, should always be done with third person obviously an specialist of the field or an easy method is to see how much exact relevant experience he has in past for the same job. I hope this would suffice.