At that time at literate people could read Latin. There were few books available to read. It only made sense for the monks to hand write Bibles in the language most people could read. Were there only languages available? Of course. The Bible was translated into German by 400AD as well as other languages. Because of cost, those who could read had handwritten Psalms if they were rich enough, the poor memorized prayers as had all generations before them. In church, the Gospel was read to the people and then explained, just as it had been from the beginning. The Catholic Church made certain that all translations, Latin and otherwise, were correct and in keeping with what has been handed down from the Apostles.
thank you
signed, a deplorable Catholic
Books were expensive because the Muslims had turned the Mederterranian sea into a sea of pirates, stopping the importation of papyrus into Europe.
That is why literacy fell so fast. It just became extremely expensive to make written materials.
http://www.frontpagemag.com/fpm/132216/how-islam-killed-greco-roman-civilization-fjordman