maternal language vs first language

maternal language and first language both are nouns.

Word	Noun	Adjective	Verb	Adverb
maternal language	Yes	No	No	No
first language	Yes	No	No	No

As nouns, maternal language and first language are synonyms defined as:

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

Other synonyms of maternal language include mother tongue.

maternal language (noun)	first 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