I think she speaks a littel english. Mostly just polite phrases etc... (she thanks the people in her Bold & Delicious pv) but i think i read somewhere that she herself said her english isn't good. For example when she had her vocal training etc... in new york before her debut she didn't actually speak any english to anyone or even leave her appartment/room much. She had a handler who did everything for her, including getting food etc.. so i wouldn't be surprised if her english was sub par.
I do remember her speaking on the mtv asia awards a few years back where i believe she said "Thankyou everybody, i love you all" after her performance.
However, even if ayu was fluent in english, i doubt she'd have success on western shores. Look at Utada for example. Exodus was a good album, but it's success (or lack of as is the case) just goes to show that sadly, asian artist have very little crossover appeal. I also believe utada's music is much more suited to a western market than ayu's, so if she can't succeed i really can't see ayu managing it.
It's a sad truth