Duolingo

The world's most downloaded language app is brilliant at one thing: getting people to open it every day. Duolingo's gamification — streaks, leagues, hearts, XP — is so effective that 37 million people use it daily. But fluency? That's where things get complicated. Duolingo excels at building vocabulary and basic grammar for beginners, but its sentence-translation approach plateaus hard around B1 level. The owl's passive-aggressive notifications are legendary. It's the gateway drug of language learning — you'll need something else to finish the job.