。
She was proficient in English.
Literal
She [topic-は] English [in-に] proficient was.
堪能 ('proficient / accomplished') is a formal な-adjective taking its domain with に — ~に堪能だ. The reading たんのう evolved from older かんのう, and the word covers polished proficiency in skills like languages, music, and traditional arts. The past copula であった is the formal/written equivalent of だった, matching the elevated tone of 堪能. In casual speech this same idea would be 英語ができた ('could do English') or 英語が上手だった ('was good at English'). Note that 堪能 has an unrelated sense ('full satisfaction'), pronounced the same — context disambiguates.