Which innovative marketing strategy did Duolingo CEO Luis von Ahn pioneer that revolutionized language learning product advertising?

In the world of language products and services, marketing strategies have evolved significantly to capture the attention of polyglots, language learners, and linguistic enthusiasts. Companies have employed various techniques to distinguish themselves in this competitive market. This trivia question explores a groundbreaking marketing approach that transformed how language learning products are advertised and perceived by consumers.

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Gamification of language learning with streaks and competitive leaderboards
Celebrity linguist endorsements featuring polyglots speaking multiple languages in single advertisements
The 'linguistic immersion guarantee' promising refunds if fluency isn't achieved in 3 months
The 'language pod' concept where advertisements featured portable soundproof booths for practice