。
She's skilled at French.
Literal
She [topic-は] French [subject-が] is-proficient.
達者 ('skilled, proficient, accomplished') is a な-adjective with a slightly old-fashioned, kindly flavor — often used to praise a senior's mastery or comment with warmth on a child's competence. Compared to 上手 (the everyday 'good at'), 達者 carries more cultivation and depth. Also used in greetings like お達者で ('take care, stay well') in old-fashioned register.