But for all the growth, there are potential speed bumps.
While the crowded language landscape includes schools, human tutors and various software platforms, many users often want to pay "little or nothing," said Carbonell of Carnegie Mellon.
Duolingo, launched in 2013, requires no subscription fees or in-app purchases, and for now features no ads.
Von Ahn readily acknowledges that "we need to make revenue." The company has started charging $20 for English certification tests. And charging for those exams and certifications—that tout language proficiency—is no accident. Such certification is a coveted among professionals, hoping to gain a leg up in multinational industries.
Plus, about 75 percent of Duolingo users are from outside the U.S. Von Ahn sees a "huge demand" for certification outside America.
Meanwhile, as the industry grows more splintered, language market leader Rosetta Stone and others have seen share prices drop.
Read MoreOK Apple Siri and Google, are you ready for Hound?
Looking ahead, growth in the language space is forecast to slow to 4.4 percent annually through 2019, down from 5 percent from 2009 to 2014, according to IBIS World.
"It's a fragmented space. Brand names like Rosetta Stone that are in there have a limited scope," said Ray Valdes, a research vice president at market research firm Gartner.
But von Ahn argues Duolingo's vast, 100 million user base helps set it apart. While dozens of students may take a single course, Duolingo's audience reach is much broader.
"I've wanted to be a teacher from the beginning. One of the things that frustrated me the most was the scale with which I operated as a teacher," he said.
Read MoreShazam, the musical unicorn with 100 million users