Duolingo is mean, whiny, and annoying. Gen Z loves it.

Alvarao Dominguez for BI

A few weeks before flying to Lisbon for vacation earlier this year, I redownloaded Duolingo. I'd used the language-learning app off and on over the years to brush up on my high-school French, but life — and weak discipline, if we're being honest — kept me from making its lessons a consistent habit. Now I hoped to pick up just enough Portuguese to dodge being branded an "ugly American." I promised myself that this time would be different because now I had the magic...