In the research project presented in the following, the role of pupils' linguistic and cultural background for learning and teaching mathematics in a multi-lingual classroom is studied. Recent results of educational research show how strongly learning success depends on the school-relevant command of the predominant language in which lessons are conducted - in Germany the German language. These results convey the impression that teaching itself does not cope with it's task to impart it's own language. In this paper a description of an empirical study will be presented - as a kind of a workshop report - in which we examine the relationship between learning/understanding of language and learning of mathematics in multi-lingual classrooms.