Catweazle is a magician from the time of the Normans who is cast into the future by magic. With the help of two boys he uses magic in an attempt to return to his own time.
Jon Pertwee was favored to play Catweazle
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.
English