Which innovative linguistic feature did Duolingo implement in 2019 that reduced subscriber churn by 13% according to their published case study?

In the world of language learning apps and platforms, subscription models have revolutionized how people access educational content. These platforms use sophisticated linguistic strategies not just in their teaching methods but also in their marketing and retention approaches. This trivia question explores how one major language learning platform leveraged linguistic psychology to dramatically reduce customer churn.

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AI-powered accent recognition that adapts to regional dialects
Personalized 'streak saver' notifications using language that triggers loss aversion psychology
Localized content featuring culturally relevant idioms and expressions
Grammar correction algorithms that use positive reinforcement language