Many studies point out teachers’ crucial role in mathematics classrooms. This role is also emphasized through the increasing need of language-responsive teaching due to language diversity in the classrooms. The paper presents a qualitative study of teachers noticing mathematically relevant language issues in a video on a whole class discussion on variables. The study investigates which language resources and language obstacles the teachers notice and how they activate didactic categories of their content knowledge. It seems to be crucial to not only know the categories as disposition but also to activate them in a content-specific way.