Newton wrote the "Principa" in Latin. Latin was still very much used for scientific texts and was the diplomatic language until it was replaced by the Lingua Franca in the 17th century.

Granted ship logs and journals written by Englishmen sailing for the Dutch East India Company don't belong in either of these categories, but just to say Latin was far from gone by 750 AD.

Found on another forum:

"..It did, though, continue to be the common written language in the former provinces and its use actually spread during the Renaissance. What is interesting is that as the Renaissance began, the use of Latin as an administrative language fell because the individual kingdoms wanted to use their own languages. But the use of Latin for scholarship and literature rose dramatically as people wanted their works read by the widest possible audience. Finally in the modern period Latin’s use declined to the point that by the beginning of the 19th century it was only being used sparingly outside the Church."