Does Asuna speak English?
Yuuki Asuna went to an all girls private school before the events of SAO. So I'm pretty sure she would have learned it at her school during that time. Most Japanese schools teach English as a curriculum. But sadly a lot of Japanese people still don't have "fluent English conversation abilities".

Considering how serious she was about clearing the game, I would guess that she probably also took her school work very seriously. And hence most definitely picked up the lot of the English phrases which she then used in SAO.