In order to become proficient in language, obtaining sufficient vocabulary is crucial. L2 learners with a home language (L1) different from the language of instruction (L2) often experience difficulties acquiring L2 vocabulary. In this regard, research points to the potential of using L1 during interactive book reading (IBR) in L2. To gain more insight into the effectiveness, comparing different approaches for using L1 in IBR thereby considering that teachers often do not master L2 learners’ L1 is necessary. This is especially relevant in primary education in view of continuous literacy promotion after preschool. Therefore, research goal 1 aims to evaluate the effectiveness of different approaches to exploit L1 during IBR in L2 to increase L2 vocabulary knowledge in 7-8-year olds. So as to transfer results to teachers, research goal 2 focuses on the first phase in teachers' continuous professional development, i.e. teacher education. To gain better theoretical understanding of pre-service teachers’ competence to apply functional multilingual practices during IBR, the relationship between their knowledge, beliefs, and teacher behavior will be examined. Additionally, the effectiveness of a pre-service teacher professionalization on these teacher competence components and on L2 learners’ vocabulary development will be investigated. In this respect, this project is treading underexplored paths by focusing on functional multilingual learning during IBR in primary education.