Which groundbreaking product design feature did Duolingo introduce in 2012 that revolutionized language learning apps and became widely adopted across the industry?

The language learning app market has transformed how people approach linguistics and language acquisition. With innovative product design and marketing strategies, these apps have created unique brand identities and revolutionized self-education. This poll tests your knowledge about a significant product design innovation that changed the language learning landscape.

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Gamification with skill trees, XP points, and streaks
Neural machine translation for real-time conversation practice
Augmented reality vocabulary recognition in real-world objects
Voice-to-text pronunciation scoring using phonetic algorithms