Oh... It's editors's horror... I'm sure it happened to a lot of us: edit horrible subtitles, so horrible that it takes more time to edit than to translate ourselves. Whether they use "google translation" or they do not use it, some are not meant to translate! And definitely, I wonder if they understand what they write. One wonders if it is indeed their native language.
I think you have to think in two steps: Prevent and wait to see if they change (editors have to suffer some time). Then, if they do not make efforts, simply remove them from the team.
It is in these situations that you also need to have a solid help document: with vocabulary, the language used ("Tu" and "Vous" for French), dictionaries... A solid and good team who can help.
But you know what? From my experience only, I did that. I waited for them to progress, I gave advice, gave feedback, suffered from an endless edition and spent an indescribable time for strangers instead of spending it with my family.
I think we can be nice for a while, but after, we are led to lose hope, not to take new ones, or even to consider them. It is sometimes better to have 3 experienced translators in a team, to translate more slowly, than to have 10 translators, including 7 new ones, and to go faster.
And you know what? We are a close-knit community, so it's good to give them a chance. So, I found a little trick, which does not please everyone. I agree to add them on the drama "On air" that they asked to translate (Yes because in addition they do it on popular dramas), but I would like to give them some advice before, if they can translate one or two parts into another drama that I give them, that I edit and with tips, the way we format sentences. It's not a judgment or a test, it's just to teach them the basics about viki, the standards we impose. But these norms are not particular norms, there are laws of the language (French in my case). It's just French, the one we learn at school. Can we vouch for it, and learn English (+ French or portuguese here), for people who should have learned it at school? I am not a teacher.
Another thing: It's your team. I like the idea of democracy, a team that helps each other with pleasure... But this is only possible with translators we know well and with whom we are used to working. If they do not integrate, do not make efforts with your advice, do not bother with them! They do not make efforts, do not make efforts for them either. Do not feel bad about firing them, now it's a dictatorship.