What innovative concept was Duolingo originally founded upon that differentiated it from other language learning platforms?

The language learning industry has been revolutionized by technology over the past decade. One company in particular has transformed how we approach language acquisition through gamification, accessibility, and innovative product design. This trivia question focuses on the origin story behind one of the most successful language learning platforms and its unique founding principle.

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Users would translate real-world documents while learning, helping to digitize the web
It was the first platform to use voice recognition technology for pronunciation practice
It pioneered the subscription model for language learning apps
It was initially developed as a government tool for rapid language acquisition for diplomats