。
She speaks English just like a native.
Literal
[topic-は] just-like mother-tongue [object-を] speak-[as-ように] English [object-を] speaks.
まるで~ように is the simile construction with ~ように ('like, in the manner of') — 'just as if speaking her native tongue.' The adverbial ~ように modifies a verb (here 話す 'speak'), while its attributive cousin ~ような modifies a noun ('a face like X,' 'a tone like X'). Same simile, different syntactic slot. 母語 ('mother tongue') is the formal term for one's native language; the casual 母国語 ('motherland's language') is also common. The compliment here is high — speaking another language at native fluency is the recognized gold standard of language learning.