。
She excels at English.
Literal
She [topic-は] English [in-に] is-excelling.
~に優れる ('to excel in/at') is a slightly formal verb — newspaper, biography, and academic register prefer it over the everyday 上手だ. The に here marks the domain or area of excellence; outside this verb, ~に優れる pairs with abstract domains: 技術に優れる ('technically skilled'), 性能に優れる ('high in performance'). The ~ている form gives the resulting state of having achieved excellence — 'is in a state of excelling.' For everyday conversation, 英語が上手だ would be more natural.