。
She mastered French.
Literal
She [topic-は] French [object-を] mastered.
習得する ('to acquire, master, learn fully') is a Sino-Japanese compound — 習 ('learn, practice') + 得 ('obtain'). The combination implies more thorough acquisition than 学ぶ or 勉強する; you've not just studied a language, you've taken it in. The plain past 習得した reports a completed achievement.