Which language learning subscription service was the first to implement an adaptive learning algorithm based on spaced repetition theory from cognitive linguistics?

Language learning platforms have revolutionized how we acquire new languages, moving from traditional classroom settings to subscription-based digital environments. These platforms employ various techniques from linguistics research while competing for market share through innovative features and pricing models. How well do you know the business side of language learning technology?

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Duolingo (2011)
Babbel (2007)
SuperMemo (1987)
Rosetta Stone (1992)