tl;dr; Combine the pros of a language buddy with chat bot software.<p>I'm in the process of learning Chinese from my wife, and love the interaction, but there is a point in time where your language partner doesn't want to repeat something to you for the umpteenth time (while that's exactly what's required to learn a language). Could a speech chat bot take over such a process? I'm thinking a lesson would be the user trying to get information out of the chat bot. Does something like this exist? Would it be feasible? An alternative like a (Text) chat bot would also work for some languages.
As a product it's a great idea, I'm sure it would sell very well.
As an effective learning tool I'm not so sure. Unless you could implement top notch speech recognition it would not do too much in improving your speech. And so far I'm yet to see English speech recognition software that would be suitable for such a purpose. The situation is even worse in case of Mandarin (and probably Cantonese too). Mandarin speech recognition that is out there is not suitable for anything more than simple voice-commands, at least last time I searched.<p>Generally speaking I'm not an advocate of practising speech before attaining a certain, relatively high level of proficiency. But that's a completely different matter.<p>A text bot from a technical standpoint would be completely doable and should be a good product as well.
You could actually build a pretty sophisticated course utilizing that text bot, a lot of people would be interested I'm sure.