Is there generally only one language in a person's mother tongue?

In fact, until now, the definition of mother tongue in linguistics is still controversial. Generally speaking, the mother tongue is the first language, that is, the first language, but if a child is born in a bilingual family (there is a four-year-old girl named Irene in Toronto, Canada. Her mother's mother tongue is Chinese, her father's mother tongue is Spanish, and her parents talk in English. According to her parents' language education plan, her mother spoke Chinese to Irene and her father spoke Spanish to Irene, so Irene took Chinese and Spanish first. After three years old, Irene mastered English again. Irene uses Chinese when talking to her mother or relatives, Spanish when talking to her father or relatives, and English when talking to her children.

For a bilingual (including multilingual) child like Irene, which language is his mother tongue depends on his parents and society's recognition of his national identity, or the child's self-recognition of his national identity. If his national identity is identified with his father, then his father's language is his mother tongue; If her national identity is to be with her mother, then her mother's language is her mother tongue. )

You said that if you speak Chinese at home and learn English at school, it is still a second language. A second language is aimed at ethnic minorities in a country.

Their mother tongue is their first language, while the national lingua franca is their second language.