Well a thousand years might be hard to judge, but we could do almost 500 years, take the Latin of Sallust writing about Cataline in the 40s BC and Ammianus Marcellinus in the 370s AD.

The actual Latin isn't much different when I was studying Latin we used both to learn the language. There are obviously some slight differences between the two writers but that's the same with Livy and Sallust write only a few decades apart.

The English language a thousand years ago was still a melting pot of Latin church writings and then Anglo-Saxon histories and Norse languages particularly Danish in the north. Of course there is still the appearance of Norman French in the ruling class.

As a follow up. How far back could someone be transported in time and still understand spoken English? When would it become too foreign?