Very cool idea, but doesn't it have a level adjustment? For example, I'm learning Chinese but my level is quite basic at the moment, it would be cool if the bot could adapt its vocabulary.
It's a suggestion for improvement but it's already very good and I think I'm going to use it a lot.
By the way, just in case you don't know and you find it interesting, in the case of Chinese there was a crowdfunding campaign about a hardware speaker that would do something like this (https://webmarketkings.com/lily-chinese-scam/). It never delivered. I was one of the backers... but well, that's life with Kickstarter and the like, sometimes.
BTW, your link doesn't work for me right now.