He'd sold two companies to Google by his mid twenties and now Luis von Ahn has created an app that he hopes will allow anyone with internet access to learn a language for free

Luis von Ahn is a man of many titles: computer science professor, entrepreneur, crowdsourcing pioneer to name but a few. He sold two companies to Google while still in his 20s and his latest creation, Duolingo, is well on the way to revolutionising language learning across the world.

"What I wanted to do was create a way to learn languages for free," says von Ahn. "If you look at language learning in the world, there are 1.2 billion people learning a foreign language and two thirds of those people are learning English so they can get a better job and earn more. The problem is that they don't have equity and most language courses cost a lot of money."

Luis von Ahn launched Duolingo in 2012 along with co-founder Severin Hacker. The app now has 40 million registered users, making it one of the most popular ways to learn languages in the world. Put another way, in America there are more people using Duolingo than learning a language through the US public school system.

So what makes Duolingo different from the hundreds of other digital language learning platforms out there? The fact that it's free is obviously a big draw, but there must be more to it than that. Von Ahn believes its success is down to a combination of clever, intuitive design aspects and constantly evolving teaching methods.

"I think the reason it's been doing so well is it's very enjoyable – there are a lot of aspects that make it a bit like playing a game – but also effective," he explains. "There's an independent study that shows that if you use Duolingo for 34 hours you learn the same as you would in one university semester of language learning."

"We've done a lot of work to improve how we teach and because of our users we are in a unique position to work out how best to teach a language. For example, if we want to know whether we should teach plurals before adjectives, for the next 50,000 users, we'll teach half of them plurals before adjectives, half the other way around and then we measure which of these groups learns better. We figure out in a couple of days which is more effective and then switch everyone to that. We are slowly figuring out the best way to teach a language, and we're doing it scientifically."

This constant datamining and huge user base means the company has an interesting overview of language trends and demographics. This means they can keep an eye on the most popular languages (currently English, followed by Spanish and French) and observe interesting snippets such as the fact that "for some reason, Italian women learn English much better than Italian men."

For von Ahn and his team, the newest development is the Duolingo Test Center, an in-app language test that came about as a direct response to customer feedback.

"What started happening is that we got a lot of emails saying thank you, but in order to get a job I need to certify that I know the language," he said.

"We started looking at the English language certification market, we realised that it is crazy and archaic – you get certified by taking a standardised test and they cost a lot of money, around $250. You also have to go to a test centre, there isn't one in every city so you have to travel, then there is a delay in getting your score. Most tests are taken in the developing world where $250 can be a month's salary, so if you want to get a better job, you have to spend a month's salary doing it.

"We decided to do something about it. The Duolingo Test Center will charge $20, you take the test from an app instead of having to travel and you get the score back within 48 hours. We're pretty excited about it."

This democratisation of language learning is obviously an issue close to his heart. The main Duolingo app will always be free and von Ahn believes that mobile represents the future of education. "A significant section of the world doesn't have access to a good education but many over the next 10 years will own a mobile phone," he says. "We see this as a way to deliver education to everyone."

• This article was amended on 9 September 2014 to clarify that Duolingo was co-founded by Severin Hacker and Luis von Ahn. The article previously suggested that the company was launched solely by Luis von Ahn.