Despite the refugee challenge, a large and growing number of students in Europe or the US come from homes where the societal language spoken in schools and daycare centers is not the primary language spoken at home (Passel, 2011; Statistisches Bundesamt/Federal Statistics Office, 2010). Language-minority students who cannot communicate proficiently in the societal language cannot participate fully in schools, workplaces, or society (August & Shanahan, 2006; Stanat et al. 2002). In particular, most dual language learners enter school with language skills and experiences that differ substantial from monolingual children (Ballantyne, Sanderman, & McLaughlin, 2008). They often lag behind their monolingual peers due to varying input and socioeconomical situations (Reardon & Galindo, 2006; Dubowy, Ebert, von Maurice, & Weinert, 2008).

While there is a growing body of evidence on language development as a key competence for learning in other domains and school success, very little is known about the specific intervention and classroom practices that foster language development of dual language learners in early childhood education settings. Therefore, a systematic investigation on the effects of these different language intervention approaches on L1 and L2 development of dual language learners is urgently needed.