ˌmother ˈtongue BrE AmE noun [countable] especially British English
your mother tongue is the first and main language that you learnt when you were a child SYN native language/tongue :
children for whom English is not their mother tongue
MOTHER TONGUE
Meaning of MOTHER TONGUE in English
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012