Which language learning product innovation first introduced the 'gamification' approach with streaks, XP points, and competitive leaderboards that transformed the industry?

Language learning apps have transformed how we acquire new languages. Behind these digital language tools are fascinating product development stories and strategic decisions. This poll tests your knowledge about a pivotal moment in language learning technology that changed the industry landscape and brought language acquisition to millions of new users worldwide. Can you identify the product innovation that sparked a revolution in accessible language learning?

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Duolingo's skill tree and hearts system (2011)
Rosetta Stone's immersion method software (1992)
Babbel's speech recognition exercises (2008)
Memrise's mnemonic flashcard system (2010)