Duolingo is a popular mobile language-learning application that has been around the market since 2012 and has been researched in its language-learning abilities. It is crucial for mobile language learning applications to keep their users interested, nevertheless, as users easily lose interest in continuing their language study. This thesis looks at how Duolingo's gamified system design implements persuasive principles to increase user engagement. The thesis aims to advance the knowledge of user engagement in language learning apps and help create useful mobile language learning applications. To achieve the goal, it was necessary to determine which persuasive principles were being applied by Duolingo's gamified system design. This was done using the Persuasive System Design (PSD) model by Oinas Kukkonen and Harjumaa (2009). The research consisted of qualitative semi-structured interviews with 8 young adults living in Sweden. The aim of the interviews was to find detailed, deep, empirical data and compare it to a theoretical framework using thematic analysis. The results of these interviews showed that Duolingo's gamified system design effectively applies persuasive principles to engage and encourage language learners. However, it was discovered that several persuasive principles needed to be improved in order to further enhance user involvement. This study adds to the understanding of persuasion by demonstrating how these principles can be applied to other mobile language learning applications to increase user engagement.