This paper focuses on the construction of language expertise in international, university-level English-medium courses where English is used as a lingua franca. Even if the courses are not language courses, language sometimes becomes the topic of discussion in the form of language correcting and commentary. This paper looks into these instances, where the teachers (i.e. subject experts) and students can be seen to take on, or be allotted, the role of language experts. The findings show that this role can be (1) based on a speaker's professional role and expertise in the relevant subject, (2) allotted to a native speaker of English, (3) negotiated between speakers, or (4) assigned to an English instructor. The paper discusses the implications of who takes on the role of language expert, and considers, in particular, to what extent the role still falls on native speakers of English. It will be shown that non-native speakers of English take on the role of language experts, and that this has implications for the kind of regulation done in the lingua franca interaction. The findings shed light on the micro-level realisation of Integrating Content and Language in Higher Education. (Contains 6 notes.)