mother tongue vs first language

mother tongue and first language both are nouns.

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

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

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

Other synonyms of mother tongue include maternal language.

mother tongue (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