Question

the use of local languages in mediavil literature was most revolutionary because

Answer 1

...the Roman Catholic Church was the dominant force when it came to education and the written word in Europe at the time which meant that if you read or wrote, it was in Latin.

It was the language of the "educated" for centuries which made any other work based on local language, like Old English or German, stand out. Even today, we still see Latin terminology used in many things from Roman Catholic Church services to the sciences.