、。
She speaks French as well as English.
Literal
She [topic-は] English [only-だけ] [is-not-でなく], French [also-も] can-speak.
The comma after ~だけでなく gives a slight pause in the phrasing — common in writing to mark a clause break and let the listener register the 'not only X' framing before the contrasting 'also Y' arrives. Without the comma, the rhythm runs straight through; with it, the structure is visually clearer. Otherwise the meaning is the same as a comma-less version of this construction.