。
She's good at English.
Literal
She [topic-は] English [subject-が] skilled is.
The textbook XはYが得意です construction. Note 英語 takes が, not を — 得意 (and its family 上手, 下手, 苦手) marks the activity or domain with が rather than を, even though the same noun would take を with the corresponding verb. The polite copula です gives a friendly, neutral tone — the kind of remark a teacher or family member might offer about her. Casual variants would be 得意だ or just 得意 with rising intonation.