Simplify the following text for a Grade 9 native English-speaking person.
Original: A first language ( also native language , mother tongue , arterial language , or L1 ) is the language ( s ) a person has learned from birth or within the critical period , or that a person speaks the best and so is often the basis for sociolinguistic identity . In some countries , the terms native language or mother tongue refer to the language of one 's ethnic group rather than one 's first language .
Simplified: A first language ( also mother tongue , native language , arterial language , or L1 ) is the language a person first learns .