Responding with my own anecdata: Duolingo is not bad, but better options exist, in with a gap in quality and quantity that you might as well just use those (admittedly, I have been learning Spanish and Japanese, languages that have a lot more resources than Mongolian).
Speaking with anyone who has intensively learnt languages, FSRS is the best way to remember vocabulary (you will need a separate source to actually learn it). And I've found that books often offer more structured practice and introduction. After you've outgrown books, picking up words from real conversations / media is much faster and makes you more likely to actually remember them.
And for Duolingo, there are many things to nitpick about. The obsessive gamification, the hearts (and the current energy) system that prioritizes getting answers correct by guesswork and cheating or being limited to a few minutes a day, the company firing chunks of it's core translation and language staff.
Duolingo is more than adequate, however, if you plan to do 5 min / day and just want to keep a streak going. I have seen people who have gotten real results from it too.
For what purpose exactly? When the computer is asked the solution to a problem, it must be absolute: "What is the sum of 12 and 45"; "Is Jack cognitively apt" etc.: we want a truth, strictly. What to do with that truth, it is another problem, and not the priority of the computer vis-a-vis the requirement from truthiness. This is, of course, still in the context of adults.
Funny that people still perpetuate the myth about boiling frogs; they absolutely jump out at a certain hot enough temperature regardless of how slowly it is heated.
> you can obviously just decide who has sex with your wife
That's your fantasies, not author's. He merely implied that one can obviously decide whom to allow to fuck one's wife. He hasn't implied that wife's consent is unnecessary or automatic.
Nope, no way to do that.
I wonder about ergonomics. I can imagine "academic" usage, but I shudder at the perspective of the impact on a typical CRUD app.