"The judge says that it is not legal."
Translation:Ο δικαστής λέει ότι δεν είναι νόμιμο.
ισχυρίζεται sounds correct to me also? I got this in a multiple choice question.
Ισχυρίζεται connotes some doubt. An accused ισχυρίζεται i.e. claims. A judge just says or tells. He does not have to convince anyone, you see, nor there is any doubt on whether what he says is true or not. (There might be some doubt about whether it is just or unjust, but this is irrelevant)