Which language learning app first implemented a 'streak' gamification feature that revolutionized user retention in the language learning industry?

The language learning app market has seen remarkable innovation and competition over the past decade. Different companies have employed various marketing strategies and technological approaches to capture market share in this growing industry. Test your knowledge about how these language learning platforms have revolutionized the way we acquire new languages and which strategies led to breakthrough success.

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Duolingo
Babbel
Rosetta Stone
Memrise