I'll second this. The vocab is probably the easiest and most straightforward bit to train right now. Flash cards have already all but solved that problem as the

I'll second this. The vocab is probably the easiest and most straightforward bit to train right now. Flash cards have already all but solved that problem as they provide the simplicity, an easily optimizable metric, and a dopamine hit from numbers going up. The part that's harder is getting people to fluent conversations, both on the comprehension and construction side.