I learned 5 foreign languages different ways, and the one I'm most proficient in, Italian, I learned the hardest way, doing grammar excercises, where for every of 31 paragraphs of the manual (each paragraph containing 2-3 grammatical phoenomena), I had to articulate ~200 sentences, each from scratch. I abandoned flashcards on the 2nd or 3rd month of learning. I also attended a discussion club, which gave that tiny bit of "coffeeshop" language the author speaks about. 1,5 years into learning, I passed CILS exam for level C (it would be C1 nowadays).

My worst language in is German, where every manual is well elaborated in terms of graphical design, but every exercise askss you to insert a word or two into a sentence. Or pick an answer from a set. Basically, Duolinguo sent to printer. So after couple of years of working with teachers and taking intensive courses, my level is B1..2. I can listen to radio and understand something, I can read something. I actually can speak in a shop -- they'll understand my level and speak accordingly -- but I can't do a normal conversation. I couldn't find a teacher that doesn't just drill you through these same fancy books.

"A friend who had been learning some language in Duolinguo and then couldn't say a sentence to a native", should be proverbial nowadays.

So, despite the app idea being interesting and compelling, this teaching approach, picking correct options from lists, are good for testing (if the subject is given little enough time), but futile at teaching.