In its late phase, Latin was incomprehensible to people from the time before the new era, i.e. from the time of the creation of Rome. But, it was still a language with dialects (like Serbo-Croatian today, i.e. you could spot if someone was from Iberia, Gaul, Italy, Libya, etc. because of their accents and words they used). Following the Great Migration and the fall of the Western Roman Empire, the language began to rapidly change because the Barbarians could not pronounce Latin fluently. The result was a quick divergence into Romance languages.
The first language that broke off was the one that is still the most different from the others – Old French. As early as 842 CE, Old French became incomprehensible to other Romanian peoples, at least to those less educated. The first document in the vernacular, “The Oaths of Strasbourg”, was written in Old French. Romanian also subsequently broke off because it absorbed 20% of Slavic words during the isolation. The languages that will remain intelligible to everyone for a long time until the end of the Middle Ages, and which will be the last to break off as separate languages, are Catalan and Provençal (Occitan), between Spanish, French and Italian. Here is a fragment: “Pro Deo amur et pro christian poblo et nostro commun salvament, d’ist di en avant, in quant Deus savir et podir me dunat, si salvarai eo, cist meon fradre Karlo, et in aiudha, et in cadhuna cosa , si cum om per dreit son fradra salvar dift, in o quid il mi altresi fazet, et ab Ludher nul plaid nunquam prindrai, qui meon vol cist meon fradre Karle in damno sit. ” French? Latin? Neither of the two.