Been living in Amsterdam and six years in still don't speak Dutch. Maybe this is the moment... How to do you ensure quality in 90 languages? Seems impossible for a one-person-show if even Duolingo is only managing to do pretty basic examples?
In terms of QA, I'm using the application to learn at least one language from every major language family, so I do reviews in about 15 language families every day.
That and constant communication with my users! :)
The quality depends on the source material you choose to give it.