maternal language vs mother tongue

maternal language and mother tongue both are nouns.

Word	Noun	Adjective	Verb	Adverb
maternal language	Yes	No	No	No
mother tongue	Yes	No	No	No

As nouns, maternal language and mother tongue are synonyms defined as:

maternal language and mother tongue: one's native language; the language learned by children and passed from one generation to the next

Other synonyms of maternal language include first language.

maternal language (noun)	mother tongue (noun)
one's native language; the language learned by children and passed from one generation to the next	one's native language; the language learned by children and passed from one generation to the next