mother tongue; native language
Refers to the language a person learns from birth, typically from their mother or family. Common in everyday and educational contexts.
Her native language is Japanese.
protolanguage; parent language
Linguistics term for a reconstructed ancestral language from which later languages are derived. Often contrasted with 祖語 (そご), which is more common in this sense.
See also: 祖語 (そご)
I practice speaking every day so I don't forget my mother tongue.
In linguistics, a protolanguage is also called a parent language.
母語 is the clear modern term for an individual's native language, while 国語 in this sense sounds more formal or old-fashioned.
native speaker
Compound of 母 (mother) and 語 (language). The protolanguage sense is a calque from Western linguistic terminology.