The company has grown to reach more than ten million active users since it was founded in 2011, and now offers 18 language courses with another 11 in production.
Duolingo previously raised a $3.3m Series A round lead by Union Square in 2011, and a $15m Series B led by NEA in 2012.
Other investors include celebrity Ashton Kutcher and author Tim Ferriss.
Last May Indian venture firm Kae Capital reportedly bought a minority stake in the parent company of language learning company CultureAlley.com.
The business provides free learning modules for self-study of languages, and charges between $15 and $20 for one-on-one online tuition with a student teacher.
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