。
She can speak Spanish in addition to English.
Literal
She [topic-は] English [only-だけ] [is-not-でなく] Spanish [also-も] can-speak.
Compact use of ~だけでなく~も to add Spanish to the implicit baseline of English. 話せる is the simple potential of 話す ('speak' → 'be able to speak'), the most efficient way to express ability with verb-internal morphology rather than the periphrastic ~ことができる. The construction here implies but doesn't strictly assert 'and English' — the ~だけでなく frames English as already established, freeing the asserted content to focus on Spanish.