。
She acquired a working knowledge of English.
Literal
She [topic-は] English [genitive-の] knowledge [object-を] acquired.
習得する ('to acquire / master') is a Sino-Japanese verb-noun, more formal than 学ぶ or 覚える. Its kanji 習 ('practice') + 得 ('attain, obtain') paint a picture of acquisition through study and practice — the path implied is sustained effort rather than a one-off fact-grab. Pairing it with 知識 ('knowledge') makes a redundancy-free formal phrasing common in academic and professional contexts. The plain past ending gives the sentence a simple, declarative feel suited to a CV bullet point or a biographical note.