It's not an excuse, it's a reality. Like how a midget actor can not play someone who is 7 feet tall.
Language has many nuances that only a native speaker can properly master. Doesn't mean that she can't do english roles, though.
It's just that she won't be terribly convincing delivering dialogue, if she's playing an english woman, yet her native language is not english.
But that point did not bother me, however pointing that out is a flaw is not the fault of the actress.