。
She speaks English fluently.
Literal
She [topic-は] smoothly English [object-を] speaks.
滑らかに is the adverbial form of the na-adjective 滑らか ('smooth') — applied here to speech to mean 'fluently.' The sensory mapping is intuitive: smooth language flows without bumps. Other adverbs in this slot would shade differently: 流暢に ('fluently,' a more formal/literary choice), ペラペラ (the colloquial mimetic for fluent speech, almost slangy), 上手に ('skillfully,' broader). The non-past 話す reads as habitual — what she generally does, the kind of thing you'd say when describing someone's abilities to a third party.