。。
She speaks French — English goes without saying.
Literal
She [topic-は] French [object-を] speaks. Let-alone English [topic-は] obvious is.
まして here pairs with 当たり前である ('it goes without saying / it's obvious') as the closing predicate. The classical-flavored copula である adds a stiff, declarative weight; the literal sense is 'as for English, it is a matter of course.' The rhetorical move is: argue from the harder language (French) to the easier (English), with 当たり前 sealing the implication that French was the test case and English follows without saying. A classroom-y construction, more often seen in textbooks than spontaneous speech.