Duolingo

Duolingo teaches languages through short gamified lessons, with streaks, points and reminders designed to make daily practice a habit.

That design solves the actual failure mode of language learning — stopping — better than it solves depth: it builds vocabulary and recognition well, and grammar explanation and conversational fluency less so.

It works best as a consistency layer alongside other material rather than as a complete method, and the free tier covers the core lessons.