Current trends in educational research focus on understanding situational processes and the dynamics of achievement emotions. In this context, the features of the learning environment and their situational impact on achievement emotions play a significant role. In this study, we consider academic language demands as a feature of the learning environment and examine how they affect boredom in a learning situation based on an instructional video. A total of 123 pre-service teachers from a German university were randomly assigned to one of three content-equivalent instructional videos that differed in linguistic complexity (easy, moderate, or difficult). Their boredom was assessed before and while watching the instructional video, as well as after completing an achievement test following the video (i.e., there were three measurement time points). A language test was also conducted to assess students’ language abilities. We found two naturally occurring groups of students with significantly different trajectories of boredom, but no different trajectories of boredom as a function of different levels of linguistic complexity. However, the fit between language demands and students’ language abilities is a key factor in determining boredom. For the linguistically difficult video, students with higher boredom had significantly poorer language abilities. In addition, students with high boredom performed significantly worse on the achievement test. Our findings highlight the importance of considering the fit between language demands and language abilities in educational settings to promote emotional well-being and positive learning outcomes.