You don't necessarily HAVE to get it translated by a professional if their English is readable. The school's understand that your recommender may not be perfect, and in fact they have no obligation like you do to have perfect English. In fact an "error-strewn" recommendation can sometimes be BETTER as it can sound more authentic! But of course, if the English is so bad that they won't understand that helps no one. The important thing though is that the recommendation be very enthusiastic!

Another option would be to work with them on a recommendation together (if this is an option) or to consult your admissions consultant to see how they can work with your recommender to get a better recommendation.