Research on assessing English as a foreign language (EFL) development has been growing recently. However, empirical evidence from longitudinal analyses based on substantial samples is still needed. In such settings, tests for measuring language development must meet high standards of test quality such as validity, reliability, and objectivity, as well as allow for valid interpretations of change scores, requiring longitudinal measurement invariance. The current study has a methodological focus and aims to examine the measurement invariance of a C-test used to assess EFL development in monolingual and bilingual secondary school students (n = 1956) in Germany. We apply longitudinal confirmatory factor analysis to test invariance hypotheses and obtain proficiency estimates comparable over time. As a result, we achieve residual longitudinal measurement invariance. Furthermore, our analyses support the appropriateness of altering texts in a longitudinal C-test design, which allows for the anchoring of texts between waves to establish comparability of the measurements over time using the information of the repeated texts to estimate the change in the test scores. If used in such a design, a C-test provides reliable, valid, and efficient measures for EFL development in secondary education in bilingual and monolingual students in Germany.