It may be a far climb to reach the ranks of Rosetta Stone, but pioneering language learning app Duolingo is taking the world—literally—by storm. In just under one year since its conception, three million users around the globe have signed up to learn any of six languages without paying a penny, either on the iPhone, iPad, or online. As of yesterday, Android users can join in the fun as well—and the jump to Google’s smartphone platform is expected to double the app’s user base.

An iPhone user myself, I’ve been using Duolingo to brush up on my basic Italian for months (admittedly inspired by the gaping holes in my vocabulary on a recent trip) and can’t recommend it highly enough. For one, it’s truly free, and not just for a trial period. The company intends to turn a profit by incorporating optional translation services into your learning; as a result, their success hinges on their teaching ability.

Since the algorithms are crafted by computer scientists—not language experts—the app is able to identify the most effective learning tools and constantly adjusts its approach to reflect ongoing data analytics. That means that whether you realize it or not, the app is learning what mistakes you’ve made and refining its lessons to help you reinforce weak skills. Plus, it’s fun: a recently added "streak" feature tracks how many days in a row you’ve practiced your skills, and each lesson nets you points (which you can use to "level up" or compete with friends via social media). That said, it’s not perfect: as can be expected from such a complex start-up product, the app sometimes misses subtleties that can frustrate learners who may not be starting from scratch. All things considered, it’s the best pre-vacation study tool we’ve yet to encounter.

Up next for Duolingo’s developers? Launching a crowdsourcing tool for users to create lessons for any language they speak (the team receives the most requests for Chinese, Japanese, Arabic, and Esperanto—yes, Esperanto—though they haven’t yet figured out how to work with a non-Roman alphabet). User-generated lessons will be kept separate as beta products until testing proves they’re ready to join the ranks of the app’s six core languages.

Nikki Ekstein is an editorial assistant at Travel + Leisure.