。
She's French by birth.
Literal
She [topic-は] birth [subject-が] French is.
生まれがフランス人 means 'her birth (origin) is French' — 生まれ functions as a noun meaning 'birth, origin, background.' The が marks 生まれ as the subject of the predicate フランス人だ. This is a standard way to state someone's nationality by origin rather than current citizenship.