The current study addresses the question whether the level of proficiency of teachers who teach a "non-native" language, English, affects their attitudes, motivation, well-being and self-reported classroom practices. This quantitative study is based on a cross-sectional research design in order to investigate the relationship between the actual English proficiency of 376 English as a Foreign Language (EFL) teachers from around the world who learned English as a foreign language and their sense of self-efficacy in the EFL classroom. Statistical analyses showed that more proficient teachers scored higher on the dimensions "Classroom practice" and "Attitudes toward students and institution". They were also more motivated and happier. Intermediate (B1-B2) teachers scored significantly lower on these measures than EFL educators with Advanced proficiency (C1-C2). No significant differences emerged between teachers at Lower advanced (C1) and Upper advanced levels (C2). An argument is made that all dependent and independent variables are connected, highly dynamic and interacting directly and indirectly, which means that causality is multi-directional. The implication is that educational authorities should organise regular in-service training to maintain and boost teachers' proficiency because investing in teachers' linguistic skills represents a long-term investment in their emotional well-being and will ultimately benefit their students.