Adult foreign language acquisition is challenging, and the degree of success varies among individuals. Anatomical differences in brain structure prior to training can partly explain why some learn more than others. We followed a sample of conscript interpreters undergoing intense language training to study learning-related changes in white-matter microstructure (FA, MD, RD and AD) and associations between differences in brain structure prior to training with acquired language proficiency. No evidence for changes in white matter microstructure relative to a control group was found. Starting values of RD, AD and MD were positively related to final test scores of language proficiency, corroborating earlier findings in the field and highlighting the need for further study of how initial brain structure influences and interacts with learning outcomes.