The paper presents the synthesis of a series of studies on support measures for pupils with additional languages in Sweden, with a particular focus on the consequences of language support measures for teaching and learning school mathematics. The analysis identifies several critical aspects in mathematics teaching when the pupils transition from one school system to another, and particularly when they do not yet have a shared language to communicate among each other or with their teachers. Three aspects with relevance to language support will be discussed and illustrated with examples from the studies: the notion of mathematics as a formal language; the notion of levels and absolute progression in mathematics; the notion that learning mathematics means understanding "concepts".