Although my recommenders speak decent English, they don't get much practice writing in English. Thus, their written English is childish and error-strewn. How do I approach this problem? Should I suggest they write the recs in their native language and have it translated by a professional? I feel like suggesting this to them will risk insulting them. And if they do this, they will have to pay a lot of money (I'll probably apply to at least 8 schools). Is there any chance of them being able to write just one letter and have it translated once for all the schools, given these circumstances?
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.
