your mother tongue

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!
See also: mother, tongue
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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.
See also: mother, tongue
Farlex Partner Idioms Dictionary © Farlex 2017
See also:
References in classic literature ?
Gliddon- and you, Silk -- who have travelled and resided in Egypt until one might imagine you to the manner born -- you, I say who have been so much among us that you speak Egyptian fully as well, I think, as you write your mother tongue -- you, whom I have always been led to regard as the firm friend of the mummies -- I really did anticipate more gentlemanly conduct from you.
Can you express yourself well in your mother tongue? (mother tongue as used here = parents' language)
"I think your mother tongue is something that shouldn't be forgotten.
"I believe that knowing your mother tongue and teaching it to future generations is important.
"Your mother tongue makes you belong to a specific country and identify with a group of people.
If you want to learn your mother tongue, this can be done; there are no obstacles in the way of that."