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reCaptcha Founder’s Language Learning Site Duolingo To Open To The Public On June 19 [online]. [cit. 2021-08-10]. Dostupné online. (anglicky)[nedostupný zdroj]
HEUSSNER, Ki Mae. Want to learn Arabic? Klingon? Duolingo turns to the crowd to create new language classes. gigaom.com [online]. 2013-10-09 [cit. 2021-08-10]. Dostupné v archivu pořízeném z originálu dne 2022-04-04. (anglicky)