By the time Luis von Ahn could complete his PhD., he had already built two companies which were later acquired by Google. While pursuing his PhD., von Ahn, along with his graduate student Severin Hacker, was interested in building something related to education.
Luis von Ahn and Severin Hacker wanted to address a problem in a specific area of education and solve it with technology. They observed that a huge demand for language education existed everywhere around the world, except in the U.S. A majority of the language learners wanted to learn English and back then, learning any language was expensive. Only the rich could afford to attend colleges and schools that offered foreign language classes, or attend adult classes. There were language learning softwares like Rosetta Stone and Open English, but again, it required one to spare ~$1000. Accessibility and affordability were the core USPs that gave birth to Duolingo.
Even before the private beta version of the app was launched, von Ahn and Hacker created a splash page explicitly stating the message '100% free'. Users could sign-up on this page to be invited for the private beta launch. During that time, Luis von Ahn gave a Ted talk about the idea of Duolingo, which received a million views, converting to 300,000 sign-ups for the private beta launch. They successfully launched the beta version in November 2011. They did not spend any money on advertising for Duolingo. The official app was launched for public on 19th June 2012. Luis von Ahn currently serves as the CEO and Hacker as the CTO at Duolingo.