Throughout most of the medieval period, most learned people and most "learning" in the sense of studies and discussion and everything around it was conducted in Latin.  The church held the texts of the scriptures very tightly as it was a great form of control for them in terms of being able to dispense the wisdom of god without any ability of the common person to find fault with it.

Writing in the vernacular signalled that authors and those who had access to the process of publishing books were considering writing in the "common" tongues of the people worthwhile and it signalled a switch for the common people to also be able to access (to some extent, it was quite a while before books were cheap enough for most people to afford and for literacy rates to begin to rise) the books of the "learned" and begin to debate them or to raise objections which was a major shift at the time.