Fromfirst +‎language.

firstlanguage (pluralfirst languages)

1. The firstlanguage one is taught tospeak; one'snative language.

   Synonyms:native language,mother tongue,mother language

   She was born and raised in France, although herfirst language is Portuguese because her parents are from Brazil.

   • 2018, Clarence Green, James Lambert, “Advancing disciplinary literacy through English for academic purposes: Discipline-specific wordlists, collocations and word families for eight secondary subjects”, inJournal of English for Academic Purposes, volume35,→DOI, page105:

     Vocabulary is a vital component of educational success in bothfirst and secondlanguage contexts.

2. The language one feels mostcomfortable andcapable with.
3. Used other than figuratively or idiomatically:seefirst,‎language.

• mother tongue
• second language

• “first language”, inLexico,Dictionary.com;Oxford University Press,2019–2022.

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