1 Department of Psychology, Concordia University, Montreal, QC H4B 1R6, Canada.
2 Department of Psychology, Princeton University, Princeton, NJ 08544, USA.
3 School of Social and Behavioral Sciences, Arizona State University, Phoenix, AZ 85069, USA.
4 RIKEN Center for Brain Science, Wako 351-0198, Saitama, Japan.
5 Department of Psychology, University of Texas at El Paso, El Paso, TX 79968, USA.

Bilingual children learn their languages through rich interactions with caregivers within dynamic family contexts. However, little is known about how families align their two languages to support bilingual acquisition and how this varies across bilingual communities. This study examines language choice alignment across two communities: French-English families in Quebec (Canada) and Spanish-English families in New Jersey (United States). Thirty-nine children aged 18-35 months and their families were video-recorded during two 20 min home play sessions-one with a primary caregiver only and one including additional household members. Utterances were coded for speaker identity and language. We found strong turn-by-turn alignment between primary caregivers and children across both communities and sessions, with observed alignment exceeding chance in ? of analyzed sessions at rates 20-22% above baseline. Other family members showed weaker correspondence with children's language choices. Children's alignment was modulated by language exposure and age, whereas caregivers' alignment only decreased when additional members were present. These findings demonstrate that primary caregivers and their bilingual children align language choices consistently across diverse family configurations and communities. This linguistic coupling may support bilingual development across diverse interaction contexts, highlighting primary caregivers' central role in early bilingual experiences across societies where bilingualism is and is not the norm.