Morton's "language knowledge for content teaching" (LKCT) is a powerful tool for understanding interactions in content and language integrated learning (CLIL) classrooms, as well as CLIL teachers' knowledge base and teacher language awareness (TLA). However, often lacking in the body of research on TLA and LKCT is an explicit stance on epistemology. We further propose an expanded construct that builds on LKCT and includes "content knowledge for language teaching" (CKLT). To address these gaps, focusing on two teachers in a Japanese context, we report on a study whose goal was developing English teachers' (n = 24) in Japanese schools knowledge base, TLA, and practices in an intervention informed by dialectical materialism. The instructor created conditions for teachers to co-construct knowledge by introducing theoretical concepts, which he invited the participants to use as a lens to understand their practices, while using their practices to build an understanding of these concepts. To trace the teachers' development, we analysed their interviews and forum interactions. We illustrate how dialectical materialist epistemology allows for simultaneously uncovering and provoking teacher development, focusing on a transformation of TLA towards the role of language as a meaning-making tool as teachers began to embrace CKLT.