Powerpoint presentation (in French) for the defense of the doctoral research work led by Rebecca DAHM at the university of Bordeaux-Segalen, France. Graduated with highest honours. This doctoral research work is embedded in the field of language didactics and is equally based on the linguistics and cognitive theoretical fields. Its main goal is to study the introduction of pluralistic approaches based on unknown languages (PAUL) within the English class, at lower secondary school. It seeks to understand the effects of such a change of knowledge on the actors of the pedagogical relationship (student and teacher). A quasi-experiment was conducted in 2011-2012 in five year 7 and four year 9 forms. Students, in groups of four, were successively confronted to three unknown languages (Dutch, Italian and Finnish). They were asked to solve metasemantic, metasyntactic or metaphonological problems in turn, for each of these languages. This doctoral work first explores the institutional and theoretical framework. Then, it presents the methodological framework so as to be able to analyze the effects of the change of the knowledge parameter which has become multilingual, both on the students and the teachers. When looking into the effects of PAUL on the Knowledge-Teacher relationship, we observe that it enables teachers to better apprehend concepts such as problem-solving, conceptualisation, learning strategies and competence. The didactic transposition is hence modified: teachers have gradually been led to develop teaching sequences with the highest standards giving more space to the student. The study of the Teacher-Student relationship highlights a change in practice, mainly due to the implementation of group work. The role of the teacher is then revised: he becomes a facilitator of the learning that is done collaboratively within the group. Finally, the analysis of the Knowledge-Student relationship underlines the necessary awareness that leads to the development of multilingual competences through the implementation of learning strategies which appear to be transferable to the study of L2.