The literature on child language acquisition, including bilinguals, is very copious. We know that babies will learn to speak two-=-or maybe more--languages simultaneously, without having an accent or mixed syntax in either one. I was raised in a bilingual home. I also know two babies who learned Chinese and English simultaneously and two who learned French and English at once. One of my grandchildren learned both Russian and English as a baby and one learned Tagalog and English, and one learned Spanish and English. Their English is perfect as i their second language. They learned to speak two languages at the same time as children learning just one.