We report on a research project that develops and measures the efficacy of multilingual teaching approaches in the early English foreign language (FL) classroom in Germany. Although there is an abundance of programmatic calls for integrating minority and other languages the students speak in FL teaching, there are few materials and methods for systematic classroom implementation, let alone evidence of their efficacy in promoting FL achievement or in raising appreciation of minority languages and linguistic diversity. We review the findings of a precursor study that systematically identified linguistic and cognitive factors contributing to early FL achievement in a longitudinal study with 200 9- to 10-year-old monolingual and multilingual students in German primary schools who have had 3-4 years of English instruction. The results show that L1/minority language proficiency and metalinguistic awareness serve as significant predictors of English vocabulary and grammar skills in both monolingual German and bilingual EFL students. Against this backdrop, we designed a teaching approach that systematically integrates linguistic comparisons with minority languages and metalinguistic awareness tasks. We implemented the teaching approach with 128 4th grade learners in four primary schools who have had two 45-min English lessons per week since the beginning of grade 3. In a pre-post-test control study design, we assess linguistic, cognitive and attitudinal effects of multilingual FL teaching. In this paper, we outline the theoretical and empirical foundations of the multilingual teaching approach, illustrate it using representative examples and activities, review its implementation from a teacher perspective and report preliminary findings of its effects on FL development.