Which gamified retail engagement strategy did Duolingo pioneer that has since been widely adopted by many retail brands to boost daily user engagement?

Language learning apps have dramatically transformed the educational product landscape, creating new retail models and customer engagement strategies. This trivia question focuses on how Duolingo—one of the most successful language learning platforms—has pioneered innovative engagement techniques that have since been adopted by many other retail and subscription-based services. Test your knowledge about this language learning giant's impact on retail customer engagement!

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The 'streak' system that rewards consecutive days of usage with virtual currency and status indicators
The 'language certification' model that charges users for downloadable achievement certificates
The 'translation marketplace' where users earn commission by translating content for retail brands
The 'peer comparison' system that publicly ranks users against others in their geographic location