What is the role of students' language background in school success within the multilingual and highly stratified education system in Luxembourg? Considering achievement differences in terms of the primary effects of social and ethnic origin, we assume that students of a disadvantaged social origin (e.g. working class), with an immigrant background, who speak languages at home other than Luxembourg's official languages show lower school achievements and are placed into lower school tracks. Analyses are based on the data of Luxembourgish primary (grades 4/5) and secondary students (grades 7/8) from two consecutive survey waves in 2016/2017 (for the international project SASAL - School Alienation in Switzerland and Luxembourg). The results indicate language background has only marginal effects, but social and immigrant origin has stronger effects. (DIPF/Orig.)