Some bounty manager or projects don't even care whether they translate it correctly or just blatantly use translation tools even if they put "no translation tools" in the rules. I've informed many BM without receiving any response, some of them even said that it is okay as the translation is not 100% the same as GT results. That's shocking and hilarious. Another reason it is hard for others, especially who aren't fluent in the language of the translation to believe it was GT result is because the translator paraphrase the result. Those scammers are pretty smart in scamming people.
I think the best course of action in dealing with these Google translator users (or any automated translators) is to gather proof and report them by making a thread about it here in reputation section; Then, DT's can tag them accordingly and they will no longer be able to participate on any translation campaign. Because if you will only inform the bounty manager about it, chances are, they will not believe you or would just ignore the information you shared. Kudos OP for sharing this info.