mother tongue vs maternal language

mother tongue and maternal language both are nouns.

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

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

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

Other synonyms of mother tongue include first language.

mother tongue (noun)	maternal language (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