ABSTRACT Decades of research has investigated the heterogeneity of English among speakers of various linguistic and cultural backgrounds, but there still remains a significant gap between how English is taught in schools in Japan and the use of the language in international communication. The “standard English” of formal education is considered to relate to ways in which students perceive kyōtsūgo (common language), as a correct and universally intelligible variety of Japanese. In light of this cross-linguistic finding, the paper explores how language diversity is conceptualised across English and Japanese by six Japanese high school teachers of English. Data drawn from interviews and correspondence were analysed using a language awareness (LA) frame. The findings revealed tension between the teachers’ understanding of language and perceptions of Japan as a “monolingual” context for English language teaching (ELT). The study has the potential to contribute to the incorporation of language diversity in ELT in Japan and beyond.