I don't know if this is a question for this Stack Exchange or for the History Stack Exchange, but I would like to know when people first understood that the Romance languages were related. I have found a 13th century text from northwestern France that possibly testifies to an awareness that various languages spoken within Europe are related to one another, but I don't know if that's particularly early. (If it is particularly early, it is also quite possible that I am misinterpreting this text.)
The text is taken from a series of glosses to the Babylonian Talmud, known as Tosafot ("additional [commentaries]"), and for this particular tractate of the Talmud (Tractate Bava Qama) they are believed to have originated in the academy of Touques, which was headed in the 13th century by a certain Rabbi Eliezer.
The gloss in question is concerned with a talmudic passage that makes reference to a dialect of Aramaic called sursi (סורסי). The gloss reads as follows:
נראה דלשון סורסי הוא לשון ארמי... והא דנקט הכא בארץ ישראל לשון סורסי ובבבל נקט לשון ארמי אור"ת לפי שמעט משתנה כעין לשון לעז שמדברים אותו לשון צח במדינה אחת יותר מבאחרת
It would seem that sursi is Aramaic... and the fact that it is called sursi in the land of Israel but Aramaic in Babylon is, according to Rabbeinu Tam, because they were a little different from one another - just like the common tongue [today] is spoken in a purer form in one state than it is in another.
- Bava Qama 83a (s.v. לשון סורסי)
Rabbeinu Tam (also known as Rabbi Yaakov ben Meir) was a 12th century scholar who is often quoted in these glosses. As with the sages of Touques, and many of the other tosafists as well, he was a French speaker. I would love it if he were referring to the relationship between mediaeval French and other Romance languages, but the word that I have translated as "state" could also mean "province". As such, it is possible (I know nothing of mediaeval France) that he is referring to different French-speaking regions instead.
If this question is suitable to the Linguistic Stack Exchange, I would love it if somebody could let me know whether or not a recognition of the relationship between Romance languages was par for the course at this point in time! And if it is not, my apologies: I will migrate it over to the History Stack Exchange instead.