Maybe the free version of the app could do the collaborative filtering part. And in the paid version you'd get the high quality content.

That lets you turn the problem of figuring out which part of your content is badly machine-translated into the problem of figuring out which of your users have enough attention to detail to spot badly machine-translated content in a language they themselves are still learning. Though I guess if you show paying users only content that lots of people think is good, at least it reduces the chances that a paying user notices an issue and complains, so it could still work in that sense.