To date, research into conference interpreting has not produced a definition of the concrete subskills associated with an A, B and C language in interpreters' combinations of working languages. Existing frameworks for performance assessment in foreign languages are not designed to cover the very advanced range of language mastery associated with an A or a B language in conference interpreting. To fill this gap and deepen our understanding of what characterises an A, B or C language, we reviewed the existing literature on language classification in interpreting and foreign language acquisition, and analysed frameworks for performance assessment in foreign languages as well as archives of feedback given to interpreting students regarding their respective languages. Based on this analysis, we developed a self-administered online test that allows applicants to assess whether their mastery of a language, in our case French, corresponds to the level normally expected for an A, a B or a C language. The test was calibrated with three different groups of subjects (native speakers of French, professional interpreters and interpreting students), showing a high level of accuracy in identifying the language classification of subjects in all samples.