My Colombian (Spanish speaking) friend married an Austrian (German speaking) friend. When the baby came, they consulted a linguistic specialist with this exact question. My friends were worried about language confusion.
Kids Learning 3 Languages
If one parent speaks to their baby in one language, and the other speaks to it in another, will the baby become bilingual or confused?
The answer: the mother should speak only to the baby in Spanish. The father should only speak to the baby in German. They should speak to each other in English. (English would be the language of parenting.). This will make the baby, if it’s a boy, about 8 months behind in English acquisition which will disappear by grade two. If the baby is a girl, there will be no lapse.
They had a boy and later a girl. The boy (left brain, academic, science and math) understands English, but doesn’t like to speak it. He speaks Spanish with his mother’s family and to her and is fluent in German. The girl (right brain, creative, musical) prefers German, ignores English, but understands it and is fluent in Spanish and German. Both do well in school.
The only confusion was when they were around four (born one year apart). They thought all women could speak Spanish and all men could speak German. So they were confused when that didn’t happen.
Both of them were trilingual learners by age three; both were trilingual speakers by age four. This method of language acquisition was effortless for the parents.