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.
He grew up overseas, but he can still speak his native language, Japanese.
Learning a language other than your mother tongue is tough.
He's good at English, but he said it's easier to speak in his native language.
She speaks Japanese like a native speaker.
After living abroad for many years, some people end up forgetting their mother tongue.