[amelius]

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.

[yorwba]

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.