mother tongue
From Longman Dictionary of Contemporary EnglishRelated topics: Languagesmother tongueˌmother ˈtongue noun [countable] especially British English SLLyour 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 tongueExamples from the Corpusmother tongue• More than one-fifth of the population had German as their mother tongue.