[godelski]

Here's an idea, but it is going to be really difficult to implement. You have lessons but at a certain point you get a chat "buddy" that only uses slightly more vocab than you know to naturally expand your knowledge. This replicates the teacher student in person discussions. You can then analyze the voice and ensure that someone has the right accent and pronunciation. Then use optimization to determine your faults and what you need to practice, optimizing memory. It isn't much different from what a human teacher would do. But I'm not aware of any of these subsystems being relatively close to human level but also not aware of much research in any of these directions. I've seen a few people try and they are just terrible.