I think the problem with ads based on the conversations is twofold:
1. Whatever you do to make it work for useful ads ("Oh, you like Goddard? Check this out on Netflix...") could also be super creepy ("Oh, you have ED? So sorry, have you tried Superdrug?") and I don't think there's any way you're going to know ahead of time. As a techie sure I see how it would be cool to have the "AI" just "say the ad" but... at least be very, very selective about what can be advertised.
2. If it works, it'll be a slippery slope. First it's our conversation about the movies, then it's movie mentions our previous conversations, eventually it's all our conversations ever, dumped into another AI to target the ads -- at which point, Facebook buys you I guess, so there's that.
If I were you I'd target the education market, make it subscription based, offer some kind of big-ish discount for public schools, and also offer some kind of specific features for language schools. For instance my teachers give me homework, why not have them seed your program with some key phrases I need to practice?
Anyway, data point of one, but if it had an iPhone app and Thai was an option, I'd pay $10/mo without thinking about it.