It's an old issue and there is no clear-cut solution.
Yes, if you have a group of people translating to one particular language, by all means create a chat group and discuss the translation, the grammar, vocabulary etc. But the competition between translators of one language can be harsh and people are not very willing to admit their mistakes.
Second, if you see something that could have come from Google translate, first check it by copy-pasting English sub into GT and then clicking translate to your language. If the outcome for 10 different subs is the same as you see it on the screen, then you know they have used it. We call the people using GT as the Evil Googlators and you need to report them to Viki, because GT is banned on Viki.
Third, Moderators are the first quality control of their language. they need to check the translation and consistency of every sub in their project. It's a completely different story if you suspect Moderators are also bad at translating. CMs, especially of the on-air shows are waaay to busy to check the quality of translation for every language in their project. That's why we have a function of Other language moderator on the project. They're supposed to whip bad Mods into shape.
Fourth, a problem arises when you ask who is going to evaluate someone's translation. When I first arrived at Viki I was surprised at the low quality of translation to my language on some of the shows. Being green and new, it never even crossed my mind to tell the CM of that project that the translation was bull****. There is a hierarchy at Viki which stops this community from evolving, whether we want to admit it or not.
What I did instead was to take over one of the projects I really loved and tried to correct the subs. But the truth is, it SUCH TORTURE to do that! It's so cruel to see your language butchered with every single subs, with every single sentence and with every single phrase.
Therefore, I don't see a systematic solution coming from our community itself. One could be developed and imposed by Viki. However, I don't think that's ever been on their list of priorities.
If you see bad translations, you could try to say that directly to the person doing it, but chances are you're going to get into a conflict which will result mostly in frustration, rather than quality improvement. You could also try to send a message to the CM. If you come across a really good one, who cares about these things as much as you, things might actually get resolved.
Sorry for the rant. You asked about something I am also very passionate about, so there it is...