noun
mother tongue; native language; language of one's country
Refers to the language of the country where one was born or raised, often implying a national language. In some contexts it can overlap with 母語 (ぼご), but 母国語 emphasizes the country rather than the individual's first language.
彼は海外で育ったが、母国語の日本語は今でも話せる。
He grew up overseas, but he can still speak his native language, Japanese.
母国語以外の言語を学ぶのは大変だ。
Learning a language other than your mother tongue is tough.
国語 means 'national language' and is often used for Japanese as a school subject. 母国語 is more personal, meaning 'the language of one's own country'.
Compound of 母国 (ぼこく, 'mother country') and 語 (ご, 'language'). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for naming languages.