That‘s probably what they thought, too. If you just pick a random bilingual person and let them proofread your learning material you are going to end up with crappy learning materials.

If you want good content, you need good editors, and they are neither cheap, nor easy to find.

The reason why companies try to save money with editors is that they don‘t „scale“ (one dev and a designer can program an interface that can be used for 20.000 different questions, but you need a lot of people to come up with those questions)

I'm not sure they've even gone that far. There are _plenty_ of long-standing complaints (on the Japanese course) that there are errors in the _English_ sentences, such as that they are grammatically wrong, or that you've translated something one way but later don't accept that as a correct answer. Just checking the comments, or proofreading the English sentences, would have caught most of them.

It's not hard to hire someone to notice that the comments are reporting errors.

Not only good editors, but good editors who are also bilingual.

Reading this thread, I'm lead to wonder if there's any open source, Wikipedia style (public editing) apps / sites / courses being developed? This would certainly combat the scaling issue, as Duolingo's comments seem to imply.

Why do you need 20,000 questions, though, when a typical user isn't going to need that many?

Engineer's disease... "why hire an expert when we're already Engineers!"