Exactly right. I teach at a uni in South Korea. Students who never lived/traveled outside the country will never, ever raise their hands if they don't understand. Not only that, but if you ask them outright, they will nod and say clearly, "yes, I understand", and then turn to their best friends and say in Korean, "I actually don't understand any of it".

This app would be a tremendous help not just to my classes (I've been here a long while, and know how to read the signs of confusion & check understanding), but also to the classes of newbies in Korea.

New teachers here sometimes go for months naively asking students "do you understand this stuff" and believing it to be true.

If Koreans could vote on this anonymously, I think they would actually do that at every opportunity.