。
She's American by birth.
Literal
She [topic-は] birth [topic-は] America is.
生まれは means 'by birth' or 'birth origin' — 生まれ (birth/origin) + は (topic). The double は structure (彼女は...生まれは) creates a topic-within-a-topic: 'as for her, as for her birth, it's America.' である is the formal copula. This is a factual, declarative way to state someone's national origin.