You might find the Dictionary of Medieval Names from European Sources a useful reference for this. It's essentially a collation of confirmed references for name uses throughout the medieval period. I know the person who runs it and can vouch for her commitment to accuracy!
https://dmnes.org/name/Mary
Guide to the structure of the entries here:
https://dmnes.org/guide
This would tend to indicate that versions of Mary have been found in Italy and Spain in Latin sources from the 900s onwards, and in all three countries in local languages from the 1200s.
https://nanowrimo.org/participants/christine-seaforth-finch
http://christineseaforthfinch.blogspot.com/