。
She was fluent in French.
Literal
She [topic-は] French [subject-が] was-fluent.
流ちょう ('fluent') used as a な-adjective predicatively, with the past copula だった. Note the kana spelling 流ちょう for 流暢 — Japanese frequently writes the second kanji of less-common compounds in kana, a typographical compromise. The が marks French as the area of fluency — parallel to the が-marking pattern with potentials and skill predicates.