This article studies the degree of L2 use in language functions in sequential bilinguals following migration, and reports on L2 internalisation patterns. Respondents consist of 149 highly educated L2-skilled Polish–English bilinguals who migrated to the UK as young adults, and experienced acculturation processes. Explored language functions cover inner speech, cognitive, and communicative functions; independent variables cover social network profile, length of stay, acculturation level, and predicted future domicile. Both quantitative and qualitative research methods were used. Respondents completed an online questionnaire and were interviewed by the researcher. The paper included the Complementarity Principle into the operationalisation and measurement of language use in bilinguals (Grosjean, 2016). The findings disclosed a consequent variation in the extent of L2 use across diverse language functions. Connections were instituted between the degree of L2 internalisation and acculturation level, social network profile, and predicted future domicile. The results displayed that sociocultural integration reliably forecasts the size of L2 use across language functions. This study adds an acculturation viewpoint to current research on L2 internalisation and language evolution across lifespan, by measuring and collating the size of L2 use across different language functions in sequential bilinguals following migration.