Assessing the impact of a heritage language intervention in preschool: a controlled trial with dual language learns from immigrant families

Purpose: This exploratory randomized controlled trial evaluated the efficacy of a heritage language intervention for dual language learners (DLLs) with a migration background. The primary objective was to assess the intervention´s impact on heritage language development; a secondary objective was to explore its effects on the languages of instruction at school. Method: The sample included 186 DLLs (48% girls) from Portuguese-speaking immigrant families in Luxembourg, where Luxembourgish is the societal and preschool language, and Luxembourgish and German serve as languages of instruction in the early elementary school years. With a mean age of 55 months at the start of the trial, participants were randomly assigned to an oral language intervention in Portuguese or an active control intervention. Assessments in the heritage language Portuguese and instruction languages, Luxembourgish and German, 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-sound knowledge - for the heritage language intervention group compared to controls. Gains in expressive vocabulary and phonemic awareness were sustained at follow-up in elementary school. Improvements were also observed in Luxembourgish receptive vocabulary and phonemic awareness. No group differences were found in German oral language and reading assessments. Conclusion: The findings highlight the efficacy of a heritage language intervention in enhancing heritage language skills and supporting aspects of development in the language of instruction. These results suggest that supporting heritage language development in preschool can promote multilingual growth without compromising school language acquisition.