Duolingo is quietly moving from a playful language app to a serious money maker.Duolingo (DUOL +0.45%) still looks like a playful language app -- cartoon owl, streak reminders, gamified lessons and all -- but 2025 marked a maturity milestone that few investors may have fully appreciated at the time.Rather than hinge on a single breakout feature, 2025 was a year the company executed at scale. User growth reached new heights, revenue crossed a significant threshold, and profitability finally expanded in a way ...