Considering no one has even mentioned her releasing an English album, it's pretty much pointless to discuss her English skills, which are mediocre, but still quite good compared to the vast number of artists in Japan who sound even worse. I don't think she has any intentions of breaking into the US/EU markets as she is most likely aware that her skills are insufficient so she most likely wouldn't be able to write the lyrics herself. end of story
On the other hand if BoA managed to record an English album and Rain did too (or at least he tried?), Ayu would be able to pull it off as well if she were ever to try ^^