、。
She can speak French — let alone English.
Literal
She [topic-は] French [subject-が] can-speak, let-alone English [topic-は] can-speak.
This sentence uses まして in a slightly idiosyncratic affirmative-extending way — 'she can do French, let alone English (she of course can do).' The classical まして normally extends a stronger case to a weaker, with the second clause often elliptical or in negative-flavor. Here both clauses are affirmative, which makes the まして reading slightly unusual; some speakers might prefer 言うまでもなく or なおさら for this. The polite ます-form throughout keeps the register consistent.