the/(one's) mother tongue
The native language of one's home country. I love coming to the area where all the expats live. Even though I am fluent in Japanese and love speaking it, it's nice to hear the mother tongue now and again. It's shameful how you have turned your back on your culture and heritage—your children don't even know their mother tongue!
your ˌmother ˈtongue
the language you first learned to speak as a child; your native language: She was born in Singapore, but her mother tongue is French.