I posted a job on this website and got a bid from a "mature and experienced translator".
I take it that was the translator's own description of his expertise. Personally, I would never take any such slogan for granted. Also, it's advisable to start with a smaller job for a first-time assignment.
He did a translation of 11 pages for me - it took the proofreader 10 hours to edit the text for publishing. When I checked I found no credentials as a translator but a degree in engineering.
First of all, it sounds as if you checked after the event - I guess you're now aware of the potential problems involved.
The issue of not having any credentials (whatever you may have been looking for) may be relevant, but not necessarily so: I have seen horrendous translations from translators with diplomas, credentials, etc.
Would you pay this engineer's invoice?
I'm afraid you will have to: under German civil law, you need to give a supplier of work a chance to rectify any shortcomings before you can reject acceptance.
What I tend to do in such instances is to first discuss the issue with the translator, and to specify exactly what was wrong with the translation. Then I offer a choice of accepting a discount (note that the 10 hours' editing are not eligible for deduction in full), or to receive payment in full, but with the certainty that there won't be any more jobs.