International audience ; Our paper deals with sociolinguistic aspects of multilingualism through the study of trans-border schooling in the geographic triangle formed by the "Nord Departement" in France and the provinces of West Flanders and Hainaut in Belgium. A number of families living in this area have made the choice to cross the border every day in order to allow their children to be educated through “submersion” schooling, a method through which they are educated in a language which is not their mother tongue, contrary to the other pupils in their class at school who share the same L1. We analyze the effects of such educational choices on family language practices and, beyond this, on the whole family structure, observing that, in some families, these effects go much farther than what was initially expected by the parents.