。
When it comes to French, she's second to none.
Literal
She [topic-は] French [in-では] anyone [to/by-にも] does-not-lose.
This is a parallel construction to the 誰にも劣らない version, but using 負ける ('lose') in its negated form: 誰にも負けない ('lose to no one'). では combines with French to set up the domain of competition. The image is competitive — she does not lose to anyone in this arena. A staple Japanese idiom for asserting excellence, used in self-promotion, character description, and competitive contexts.