The first years of life represent a unique window of opportunity for foreign language learning. However, key questions are: How much and what kind of foreign language exposure is needed to ignite learning? We conducted a foreign language (English) intervention in four public Infant Education Centers in Madrid, Spain. Intervention children (N = 126, ages 7--33.5 months) experienced 18 weeks of daily, hour-long, group English sessions with native-speaking tutors, using a brain-based method of infant language development. Intervention infants were compared to a matched Current Practice Comparison (CPC) group of peers in the same schools. Intervention children outperformed the CPC group, showing rapid gains on measures of English word comprehension and English speech production. Schools' neighborhood wealth was not a significant factor in learning. Follow-up analyses show that the language gains were fully retained 18 weeks post-intervention. Children's foreign language skills advance quickly in school using this research-to-practice curriculum.