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Language mixing is a common feature of bilingual communication, yet its predictors and effects on children's vocabulary development remain debated. Most research has been conducted in contexts with clear societal and heritage languages, leaving open questions about language mixing in environments with two societal languages. Montreal provides a unique opportunity to examine this question, as both French and English hold societal status, while many families also maintain heritage languages. Using archival data from 398 bilingual children (7-34 months), we looked at French-English bilinguals (representing societal bilingualism) and heritage-language bilinguals within the same sociolinguistic environment. We assessed the prevalence, predictors, and motivations of parental language mixing and its relationship with vocabulary development. Results revealed that mixing was less frequent among French-English bilinguals compared to heritage-language bilinguals in the same city. The direction of mixing differed between groups: French-English bilinguals mixed based on language dominance, while heritage-language bilinguals mixed based on societal language status. Primary motivations included uncertainty about word meanings, lack of suitable translations, and teaching new words. Mixing showed minimal associations with vocabulary size across participants. These findings suggest that parental mixing practices reflect adaptive strategies that vary by sociolinguistic context rather than detrimental influences on early language acquisition.