。
She's proficient in French.
Literal
She [topic-は] French [subject-が] is-proficient.
堪能 (たんのう) ('proficient, accomplished, fluent') is another な-adjective for skill, often used specifically of language ability. Compared with 達者 it has a slightly more formal, evaluative weight — common in resumes, news reports, and biographical sketches. The same が-with-skill-predicates pattern applies: French is marked with が as the area of proficiency.