Or maybe more like modern English?
Certainly, it's a language that unites communication of many diverse groups. My question is what kind of people spoke each language. Judging by Gandalf's comments in Rohan, one would expect people even in low ranks to speak Common if their job has any interaction with foreigners. A village with no external trade and no road with travellers nearby might not have Common speech though. How widespread or restricted was Latin in the Middle Ages? Was ot a language of the well educated, or would other classes master it as well?