。
She knows English inside and out.
Literal
She [topic-は] English [in-に] is-well-versed.
精通する ('to know thoroughly / be well-versed in') is a Sino-Japanese verb of high register, almost always used with に for the domain of expertise. Its kanji are 精 ('detailed, refined') + 通 ('pass through, expert in') — literally 'thoroughly pass through,' the image of someone who has gone all the way through a subject. ~ている marks the resulting state: she's in the position of being thoroughly knowledgeable. Strong word — using it for 英語 implies more than fluency, closer to expertise on linguistics or literature.