This longitudinal study explores cross-language transfer among children attending multilingual public schools in Luxembourg, where Luxembourgish is the instruction language in preschool and German the language of instruction in elementary school. Following 132 children from preschool (age 5) to grade 2 (age 7), annual assessments were conducted to evaluate their vocabulary proficiency in both Luxembourgish and German. Bilingual learners were systematically classified based on theoretically motivated criteria, encompassing home language background and Luxembourgish language proficiency. Longitudinal analyses were carried out employing linear mixed models and multigroup path modeling, targeting both monolinguals and various groups of bilingual learners. The results provide robust support for the presence of cross-language transfer between Luxembourgish and German. Notably, Luxembourgish vocabulary proficiency in preschool emerged as a significant predictor of subsequent German proficiency for both monolingual and bilingual learners. Furthermore, bilingual learners with lower Luxembourgish vocabulary in preschool exhibited compromised German vocabulary in elementary school. These findings underscore the importance of cross-language transfer theories and their implications for multilingual education practices in Luxembourg.