first language vs maternal language

first language and maternal language both are nouns.

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

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

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

Other synonyms of first language include mother tongue.

first language (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