。
She's quiet by nature.
Literal
She [topic-は] innately quiet is.
先天的 (せんてんてき) means 'innate' or 'congenital' — literally 'before-heaven-like,' a Chinese-origin compound contrasting with 後天的 (こうてんてき, 'acquired'). 無口 (むくち) means 'taciturn' or 'quiet' — literally 'no mouth,' describing someone who doesn't talk much. The phrasing 先天的な無口 is stiff and somewhat clinical; a more natural way to express this would be 生まれつき無口 or 元々おとなしい.