Although there is a body of research on the teacher first language (L1) use in English-medium-instruction (EMI) classrooms, very little research has probed into how international students perceive local teacher L1 use and their learning practices in the English as a lingua franca (ELF) context. To address the gap, the present case study explored a cohort of Chinese international students' attitudes toward German-native teacher's L1 use in EMI classes at two universities in Germany. A mixed-methods approach was adopted through a student questionnaire and semi-structured interviews with Chinese postgraduate students from different majors. Results of quantitative and qualitative analyses revealed the attitudinal complexities of participants toward local teachers switching to German in EMI. Besides, Chinese students employed different learning strategies to deal with language-related challenges. Notably, participants highlight English as the most effective lingua franca. Nonetheless, they allow German to be conditionally used in certain circumstances without causing comprehensibility problems, such as telling German jokes, chatting with German students, or talking about trivial topics. Furthermore, the multifaceted understanding of language uses by Chinese international students offers insights to conceptualisations of language and EMI policies.