。。
She can speak French — let alone English.
Literal
She [topic-は] French [object-を] can-speak. Let-alone English [topic-は] all-the-more is.
なおさら ('all the more, even more so') extends a claim from a baseline to a stronger case. The まして ... なおさら pair is a classical rhetorical move: 'X is true; let alone Y, even more so.' Here it argues from French (taken as harder, more impressive) to English (taken as easier or more expected) — 'if she can manage French, of course English is no problem.' The logic depends on French being treated as more difficult, which in Japan it generally is: English is taught extensively from grade school; French is a niche specialty. The structure creates a quietly admiring tone.