With the widespread use of portable digital devices, especially smartphones, learning a new language has become a popular trend in the age of globalization. Amongst mobile-assistant language-learning applications, Duolingo is widely used and employs persuasive design techniques to inspire and encourage users to acquire new linguistic skills. Several papers have been studied based on this application by exploring pedagogical issues and discussing the misuse of gamification design. From a holistic perspective, limited research exists on how Duolingo's persuasive design motivates and engages language learners. The study utilized a walkthrough and a focus group alongside semi-structured interviews with 11 Duolingo users to investigate how persuasive design elements are employed to engage users. The findings of this study suggest that the gamification design in Duolingo is enjoyable and enhances users’ motivation; streaks make users commit to their learning plan; rewards encourage users to learn constantly; notifications play a significant role as triggers for learning behavior to occur; notifications also remind users to return to the application after a suspension. Moreover, it is important to design notifications with both the attitude and content in mind, as they have the power to affect user experience.