Which innovative marketing strategy did Duolingo implement in 2019 that dramatically increased user engagement and changed how language learning products are promoted?

In the competitive world of language learning products and translation services, branding strategies have evolved significantly to connect with global audiences. Major companies have employed fascinating marketing approaches to differentiate themselves and communicate their unique value propositions. This trivia question explores how one prominent language technology company transformed its marketing strategy in a way that revolutionized the industry.

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The 'Streak Society' gamification system that rewards consistent daily practice with exclusive benefits and social recognition
The 'Language Immersion Guarantee' that promised refunds if users didn't achieve conversational fluency within 3 months
The 'Global Translator Partnership' that paired users with native speakers for authentic conversation practice
The 'Linguistic Heritage Campaign' that donated to language preservation efforts based on user activity metrics