How Duolingo Mastered the Fickle Language of Startup Success Luis Von Ahn, co-founder of the popular free language-learning app, opens up about his passion project's past, present and future.

This company was included in our Entrepreneur360™ Performance Index coverage.

Speaking a second language is like possessing a master key that unlocks limitless doors of opportunity. Milestones that might otherwise be impossible to achieve using your mother tongue alone: A new country. A new job. A new life. An escape from poverty.

A firm belief in multilingualism as a pathway to success is what fueled Carnegie Mellon computer science professor Luis Von Ahn, 36, and his former Ph.D. student, Severin Hacker, 31, to launch Duolingo. The Pittsburgh-based startup created a popular free app, also called Duolingo, that makes learning a new language all fun and games. Literally.

Related: Duolingo, the Chart-Topping Language App, Unveils a Platform for Teachers

Turns out the app was an accident of sorts, a "happy mistake." Back in 2011, Von Ahn and Hacker originally set out to launch a basic language education website. The mobile app was an afterthought, an "extra feature" that's taken off beyond their wildest dreams. (Of course, a few star-powered hefty investments along the way from the likes of Ashton Kutcher, Tim Ferriss and Google Capital didn't hurt.)

Today, Duolingo puts foreign words on the tongues of some 15 million monthly active users. This includes people in CEO Von Ahn's native country of Guatemala, where the inventor-entrepreneur says most residents can't afford a good education.

"I've always thought in order to get out of poverty, people need education and yet having access to a good education requires money," he says. "So we decided to create a high-quality education platform that would be free for everyone, and we chose to focus on teaching something that could really make a difference in a person's life in terms of salary potential."

Related: What Type of Connector Are You? Here's Why It Matters to Your Business.

Proud to launch UberEnglish in Colombia, a new @uber view with English-speaking drivers certified by the Duolingo Test Center English exam!

— Duolingo (@duolingo) September 9, 2015

The app, available for Android and iOS, offers gamified courses in more than 40 languages in all. Users engage in colorful, adorably illustrated multiple choice and fill-in-the-blank mini-games that test their reading, writing and speaking skills. They can also compete against friends, earn badges and post high scores on social-media platforms.

All the while, they chip away at translating the Internet. That would be the whole Internet. One translation game at a time, users help Duolingo translate existing online content, everything from news articles to Creative Commons and Wikipedia entries and beyond. Consider it a pay-it-forward tradeoff for acquiring new lingo for free.

Not only do Duolingo users learn to talk the talk at no financial cost, they also don't see a single ad. No in-app purchases either. This strong focus on its customers helped the brand land on this year's Entrepreneur360™ Performance Index, a collection of the best entrepreneurial companies in America.

Related: How to Create a Multilingual Website

So how does the startup stay afloat? Von Ahn says it generates revenue via profits earned from its Test Center, which went live last year. For $20, anyone can take a 20-minute test on the website that certifies their language skills for educational institutions and employers. The offering directly competes with a similar test, the standard Test of English as a Foreign Language (TOEFL) exam, which costs up to $250 to take.

The company's recent expansion into language testing is apparently already paying off, with Uber now using its services to vet employees for English proficiency in Columbia, for example. The Harvard Extension School, LinkedIn, Upwork and Novell are also now accepting Duolingo English fluency test scores.

Meanwhile, Duolingo is catching fire in the classroom, modernizing the way kids learn foreign languages on a massive scale. So far, more than 100,000 teachers across the globe have signed up to use Duolingo For Schools, Von Ahn tells us. He says the government of Colombia has even adopted the program for official use in public schools, as has the government of Costa Rica, along with parts of Brazil and Guatemala.

As for what's up next, Duolingo, fresh off a $45 million funding round, is focusing on machine learning, a segment of artificial intelligence, as a way of automatically personalizing each learning encounter. "The goal is to offer the experience of having a tutor for everyone, for free." Ah, progress. We like the sound of it in any language.

Related: A World of Customers Is Waiting to Read Your Website in Their Language