noun
one's native language; one's mother tongue
Refers to the language of one's own country; often used in contrast to foreign languages.
海外に住んでいると、自国語を忘れそうになることがある。
When living abroad, there are times when you feel like you're forgetting your native language.
彼は自国語のほかに三か国語を話せる。
He can speak three languages besides his mother tongue.
母国語 is a near-synonym but may carry a slightly stronger emotional tie to one's homeland.
Compound of 自国 (one's own country) + 語 (language).