Assessing the Impact of a Heritage Language Intervention in Preschool: A Controlled Trial with Dual Language Learners from Language-Minority Backgrounds

Purpose: This exploratory randomized controlled trial evaluated the effectiveness of an oral language intervention delivered in the heritage language for dual language learners (DLLs) whose home language differs from both the school and societal languages. The primary objective was to assess the intervention's impact on home language development and its transfer to second language skills.Method: The sample included 186 DLLs (48% girls) from Portuguese-speaking families in Luxembourg, where Luxembourgish is the societal language. With a mean age of 55 months at the study’s start, participants were randomly assigned to an oral language intervention in Portuguese or an active control intervention, both administered in preschool settings. Language assessments were conducted immediately post-intervention and nine months later.Results: Immediate post-intervention assessments showed significant gains in Portuguese language proficiency, including vocabulary, narrative skills, phonemic awareness, and letter knowledge, for the oral language intervention group compared to controls. Transfer effects were observed in Luxembourgish, with improvements in receptive vocabulary and phonemic awareness. Gains in Portuguese vocabulary and phonemic awareness were sustained at the nine-month follow-up.Conclusion: The findings indicate that school-based oral language interventions can enhance heritage language proficiency and facilitate skill transfer to specific domains of a second language. These results suggest that targeted preschool language support may benefit DLLs in developing both home and school language skills.