。
Of course she can speak English.
Literal
[topic-は] of course English [subject-が] can-speak.
もちろん ('of course') is an adverb borrowed from the Sino-Japanese 勿論 — even though it's overwhelmingly written in kana now, the kanji literally read 'must-not discuss,' giving the original sense 'no need to argue.' The potential form 話せる takes が rather than を for its object, since potential predicates pattern grammatically with stative verbs (like できる, わかる) — 'English [subject-of-being-speakable] is doable.' Beginners who are used to を-marking direct objects often slip with this; the が-marking with potentials is one of the first big particle resets.