Duolingo(1)Language service Duolingo has just closed a US$45 million of Series D funding led by Google Capital with participation of existing investors at a valuation of US$470 million, raising the company’s total funding to US$83.3 million.

The company’s previous investors include Union Square Ventures, NEA, Kleiner Perkins Caufield & Byers, as well as individual investors of actor Ashton Kutcher and author Tim Ferriss.

Founded in June 2012 by reCAPTCHA inventor Luis von Ahn, Duolingo now claimed has gained more than 100 million users around the world by integrating gamification elements introduced for the learning process.

“Duolingo’s mobile-first, adaptive, and gamified platform is changing the way people are learning languages across the globe,” said Laela Sturdy, partner at Google Capital. “We were blown away by Duolingo’s growth than engagement numbers, and we’re thrilled to partner with them as they shape the future of education.”

Before Duolingo, Luis von Ahn has sold two projects to Google. ESP Game, which von Ahn launched in 2005, was acquired by Google. The search giant latter licensed it and turned it into Google Image Labeler which would be used for improving the accuracy of the image search. Von Ahn also sold identity verification device reCAPTCHA to Google in 2009.

Positioned itself as a global language learning service, Duolingo also set eye on China’s huge market. The company first launched a Chinese version for users to learn English in May 2014 and had attracted 1.5 million users in the first week. China is also included as an important part of its recent launch for Duolingo for School, a platform allowing students do extra work in game-like interface and teachers to optimize in-class time by addressing general questions. Duolingo has registered more than 4 million users in China as of Jan. this year, according to Ling Ma, head of Duolingo China.

Like many other foreign services, Duolingo has experienced some tough time in tapping Chinese market due to the unstable access to servers in the country. The company’s service has been partially blocked for a few days last May. Similarly, MOOC site Coursera also faced slow access to its video courses before finding local partner Guokr, a science networking service.