The growing trend towards the internationalisation of education has prompted many institutions to adopt English as a Medium of Instruction (EMI) in their curricula and instruction. This study investigates the beliefs, challenges, and pedagogical practices of school teachers using EMI to teach various subjects in Thailand. Data were collected through interviews with 78 Thai content teachers across different disciplines. Content and thematic analysis revealed the complexity and diversity of EMI teachers' beliefs, which significantly influence their engagement with and implementation of EMI in their teaching practices. As these teachers attempted to translate their beliefs into practice, they encountered challenges such as limited English proficiency and difficulties in grasping subject-specific terminology, impacting students' understanding of the subject content. Effective strategies identified to address these challenges include scaffolding techniques, L1 use, code-switching, and translanguaging, albeit potentially conflicting with the goal of promoting English proficiency. The study argues that while the use of L1 and translanguaging may appear contradictory to EMI practices, it remains crucial for EMI implementation, particularly for teachers and students facing English proficiency challenges. The findings also indicate that EMI teachers possess adequate English language skills for effective EMI instruction, and collaboration among teachers, administrators, and practitioners is crucial.