。
She speaks French as her native language.
Literal
She [topic-は] French [object-を] mother-tongue [as-として] speaks.
として ('as, in the capacity of') is a versatile particle marking role, function, or status. Here it slots in to describe how the language is used — 'as her mother tongue.' 母語 ('native language, mother tongue') is the technical term in linguistics; 母国語 ('language of one's country') is a related but distinct concept covering national rather than mother-spoken languages. The sentence reads cleanly as a biographical or linguistic statement.