The language of one's country or the language a person learned first; mother tongue or native language.
私の母国語はスペイン語です。
My mother tongue is Spanish.
留学中でも母国語を忘れないように母国語で話す時間を作っている。
Even while studying abroad, I make time to speak my mother tongue so I don't forget it.
子どもは母国語を家庭で自然に覚えていくことが多い。
Children often acquire their mother tongue naturally at home.