Jon Pertwee was favored to play Catweazle
Salmay, Dalmay, Adonay!
The English language at the time of the Norman invasion was almost completely different from the version Catweazle speaks. His style of spoken English is from at least 300 years later, when English was moving towards a style that is starting to become familiar and a construction more in line with the Germanic languages even though some of the words have evolved from the French language the invading Normans spoke. So in essence, unless Catweazle has learned to speak the English of the middle ages overnight, he should not be able to understand or make himself understood without a huge amount of difficulty. Plus his ability to read the English of the 20th century should also be more or less non-existent. However as this is a childrens programme, you can understand the writers decision to overlook this issue.
Take a look back at the TV series that took home Golden Globe Awards for Best Television Series in the categories of Drama and Comedy.