Malcom Douglas said:

"Hwæt we Gar-Dena" etc...   

Which is Old English (Anglo Saxon). All the available evidence points to Arthur being a leader of the BRITONS, who were subject to a hostile takeover bid by the Saxons just then. His language as others have said would be a forerunner of Welsh when he wasn't speaking bad Latin.

" Arthur, if he existed, was probably too busy fighting the Danes"

Never the Danes. Just the saxons, Angles, Jutes and of course other Britons (oh, and dragons, green knights, and rogue elements of Al Qaida)