Last week Duolingo CEO Luis von Ahn announced that the free language learning app was launching Duolingo Plus, a premium version for Android users. For $9.99 a month, Duolingo Plus will offer an ad-free experience and allow users to download language lessons for offline use. The company will continue to offer its free, ad-supported version for language learners around the world.

For now, the premium product will not be available for iOS users but that’s coming in the future, says von Ahn. The new product comes after months of testing different monetization opportunities such as ads and optional in-app purchases. The goal was to find a way for Duolingo to remain free for many, but to become self-sustaining into the future. Duolingo Plus is the solution.

“We decided to introduce Duolingo Plus in response to many user requests for offline usage and an ad-free experience on Duolingo. Being able to download lessons is particularly appealing to those learning a new language for travel purposes who want to practice while flying or without a stable Wi-Fi connection. It is also useful for underground commuters who play Duolingo on their way to work or school,” von Ahn writes.

“This is the next step toward making Duolingo fully self-sustaining so that we can continue to grow and provide accessible education opportunities for millions,” the CEO adds.

Duolingo was launched in 2011 by von Ahn, a professor at Carnegie Mellon University and the man who created CAPTCHA, and Severin Hacker. The app has more than 120 million registered users and more than 6 billion exercises are completed each month. It is available for iOS, Android and Windows 8 and 10.

Languages are taught in bite-size courses where students learn how to read, write, listen and speak different languages, using a game-style format. Learners lose “lives” when they get something wrong, and they get points when they complete lessons. Duolingo also tracks the number of days a user logs in in a row, so they can keep track of their learning “streak.”

Duolingo, which was named iPhone App of the Year in 2013 and Google’s Best of the Best in 2013 and 2014, offers 68 different language courses and in 23 different courses including Spanish, French, German, Italian, Portuguese, Russian, Dutch, Swedish, Irish, Turkish, Danish and more.

Insider Take:

The new premium service goes against the initial promises made by Duolingo in this video published October 28, 2013 which says no annoying ads, no misleading in-app purchases and no subscription fees.

However, the company is still offering a free version to registered users, and it makes sense that the company needs to find a way to generate revenue. We don’t think anyone can fault them for needing to support their own endeavors, and they aren’t taking away anything from current users. Instead, they are adding premium features for users that want them. We also love that they tested different options before arriving at a solution. As they roll it out, they may find they need to make some other tweaks or changes.