The recent trend of the internationalization of higher education has increased the significance of English as a medium of instruction and communication on campus in non-English speaking countries. Within this context, this study explores emotional vulnerability of teachers of Korean as a second language (KSL) over their foreign language competency, particularly English. Analysis of interviews of twelve KSL teachers demonstrates these teachers' divergent ways of interpreting and implementing Korean-only instruction, a prevalent norm within the language program, according to their perceived foreign language competency and relevant emotional vulnerability. KSL teachers with proficiency in other foreign language(s) tended to challenge the monolingual norm by utilizing their bilingual skills and experiences as resources for their teaching. Monolingual KSL teachers interpreted Korean-only narrowly and supported a monolingual immersion approach as a way to secure their teacher authority. Regardless of their attitudes towards the Korean-only instruction, however, most teachers experienced various levels of anxiety concerning their perceived lack of adequate English proficiency in the KSL classroom. The results suggest how second language teachers struggle to maintain legitimacy and authority against the hegemony of English in non-English second language contexts, providing implications for the language teacher education.