Wikipedia's article Indiana Jones and the Last Crusade | Plot says the action is set in 1938, and that the knight had been kept alive for 700 years (therefore born ca 1200), making him a speaker of Early Middle English (as opposed to the modern Received Pronunciation he speaks in the film).

This post -- and the threads linked at the bottom! -- discusses mutual intelligibility between Modern English (which Indy speaks) vs the language at various points in the past; look out for comments about Middle English, specifically Early Middle English.

• I am a native English speaker with a hypothetical time machine. How far back in time in England can I go and still communicate intelligibly with the locals?