Pre-service teachers of all subject areas need learning opportunities in the field of German as a Second Language (GSL) in order to adequately facilitate second language learners in the content classroom. On the basis of a competency model including the sub-competencies Subject-specific registers, Multilinguaiism and Didactics, the BMBF-project DaZKom has conducted a validation study. This included developing a test instrument with 68 items and analyzing it with IRT-methods. The results of the validation study (N = 252) show the expected correlations between GSL competency and linguistic knowledge as well as pedagogical knowledge, yet that test measures a conceptually independent construct. Furthermore, it is found that pre-service teachers with a higher number of learning opportunities and semesters, and with German as a study subject, show a higher GSL competency.