This person-centered study examines the extent to which parents' language dominance influences the effects of an after school, multi-family group intervention, FAST, on low-income children's emotional and behavioral outcomes via parents' relations with other parents and with school staff. Social capital resides in relationships of trust and shared expectations, which are highly dependent on whether parents share the language of other parents and teachers. This study is based on a community epidemiologically-defined sample of Latino families (N = 3,091) in San Antonio, Texas and Phoenix, Arizona. Latent profile analyses revealed three language profiles of parents across the two cities: English-dominant, Spanish-dominant, and bilingual. Path models revealed that FAST did not have a direct or indirect effect on children's emotional and behavior functioning, although FAST increased parent-parent and parent-school social capital among Spanish-dominant parents in Arizona and these parent-parent relations were associated with better child outcomes. Implications for interventions are discussed.