It's a project born out of guilt from the man behind one of the most annoying features of web surfing - those online security checks involving random words.
Duolingo hopes to convince millions of people to work for free and thus translate all web content in a matter of years.
It may sound like an ambitious plan but it's not the first time founder Luis von Ahn and his colleagues at Carnegie Mellon University have enlisted a global workforce to work for nothing.
As a 22-year-old graduate student in 2000, von Ahn invented the Captcha - those distorted images of words and numbers used to sign in to ticketing and social media websites, among others, which users have to decipher to prove they are human.
The software is used by more than 350,000 websites to prevent computer programs from attacking them with spam. In 2007, von Ahn realised that 200 million Captchas were being typed by people all over the world every day.
"At first I felt really good about that because I thought, 'Look at the impact that I've had'," he says. "But then I starting feeling bad."
Typing each Captcha takes about 10 seconds, he estimates. Multiply that by 200 million, and humanity as a whole is wasting about 500,000 hours on these security codes every day.
He decided to put these hours to good use and devised ReCaptcha, a system that uses each human-typed response as both a security check and a means to digitise books one word at a time.
At the same time the New York Times was digitising 156 years of its archive using a team of typists. Over a decade, the typists had transcribed 27 years of newspapers. The paper began using von Ahn's software and in 24 months had transcribed the remaining 129 years of archived newspapers.
ReCaptcha was acquired by Google in 2009, and it is still used widely to tell humans and spamming programmes apart. But its translating software is exclusively available to Google's Books project to transcribe every book in the world.
All of that doesn't detract from the fact that for most people, these security codes are nothing more than a frustrating waste of time. For those with dyslexia or sight problems, they can be a serious barrier to internet use.
Dr Sue Fowler, at the Dyslexia Research Trust, says the codes only add to the trouble dyslexics have filling in web forms. "Even looking at it closely, I wouldn't know what to do with it," she says.
There is an audio alternative, but these are even more confusing as most just sound like a flurry of noise.
And the automated security codes are getting more and more difficult. Some of the latest manifestations can appear as a jumbled blur of letters, numbers and punctuation that is almost indecipherable.
"As of a few months ago, if we showed someone a ReCaptcha they were successful at it about 93% of the time," says von Ahn, adding that once that drops to 75%, users give up on trying to access a site.
Since selling ReCaptcha, von Ahn has teamed up with one of his graduate students, Severin Hacker, to create software that gives the user something in return for their time and effort.
The answer is Duolingo, a site that gives free language tutorials and in exchange solicits aspiring linguists to translate sentences from the internet.
At present, it only caters for English speakers looking to learn French, German or Spanish, and Spanish speakers who want to learn English. They start with very simple sentences and work up towards more complex ones, increasing their value as a translator as they progress.
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