Russian Scottish Gaelic Spanish Swahili Swedish Turkish Ukrainian Vietnamese Welsh Yiddish Zulu A course on Xhosa for English is in the works; it will be released in 2024. Duolingo ABC Duolingo ABC is a mobile app for young children for learning letters, their sounds, phonics, and other foundational early reading concepts that was released in 2020. Duolingo Math Duolingo Math is a mobile app for learning mathematics in a form similar to the original app, released in October 2022. It is currently only available on iOS devices. Duolingo English Test Duolingo English Test (DET) is a standardized test of English language proficiency for non-native English language speakers. Business model Most language-learning features in Duolingo are free of charge, but it has periodic advertising in both its mobile and web browser applications, which users can remove by paying a subscription fee or promoting referral links. This feature is known as "Super Duolingo" and includes benefits such as unlimited hearts (retries), level skipping, no ads, and progress quizzes. Duolingo had a revenue of $1 million in 2016, $13 million in 2017, $36 million in 2018, and $250.77 million in 2021. In May 2022, it was reported that 6.8% of Duolingo's monthly active users pay for the ad-free version of the app. Duolingo derived most of its revenue from subscription, advertising, and its Duolingo English Test. In April 2020, Duolingo passed one million paid subscribers. In March 2022, it had 2.9 million paid subscribers. Infrastructure Duolingo utilizes the Amazon Web Services (AWS) product suite, including Amazon DynamoDB, Amazon Virtual Private Cloud, nearly 200 virtual instances in Amazon Elastic Compute Cloud (EC2), Amazon Simple Storage Service (S3) and Amazon Relational Database Service (RDS). The website's backend is written in the programming language Python. A component called the Session Generator was rewritten in Scala by 2017. The front end was written in Backbone.js and Mustache but is now primarily in React and Redux. Duolingo uses Amazon Polly for voice synthesis. Duolingo provides a single-page web application for desktop computer users and also smartphone applications on Android and iOS App Store platforms. 20% of traffic comes from desktop users and 80% from mobile app users. Reception Effectiveness A 2017 study found no significant difference between elementary students learning Spanish through the "gamification" of Duolingo and those learning in classroom environments, with both groups demonstrating a similar increase in achievements and self-efficacy. A 2022 study on adults