I built something like this for Mandarin Chinese at a company I worked for in Shanghai, but the company was acquired and sort of put out to pasture, and it never launched. :-(

Essentially, we took already word-segmented dialog (splitting Chinese sentences into individual words is non-trivial, so having it already segmented was super useful), matched it to words that you knew, and suggested the next lesson you should learn by the percentage of vocabulary that would be new or challenging for you. It was pretty awesome, would love to have another shot at it someday.