The Council of Tours, in 813, directed priests to give their sermons in the "simple Roman language," as they put it, a recognition that the language spoken by the common people was different enough from the Latin used by the Church as to no longer be intelligible. The Oaths of Strasbourg in 842 were written down in Latin, Old High German and an early form of French, and are the earliest text in a language that led to modern French.

Latin continued to be the usual language in writing and the international lingua franca, so to speak, for hundreds of years after the fall of the Roman Empire, which makes it difficult to date precisely -- by the time writing in the Romance languages became common, those languages would have already been established for probably hundreds of years.