The game is in Modern English, just an antiquated form of it.

Elizabethan English, which is what Shakespeare wrote in, was Modern English.
Old English was the language of the Angles and Saxons (Anglo-Saxon) and is more Germanic than what we consider to be English today. The untranslated version of Beowulf was written in Old English.
Middle English was the in-between version when the speech of other predominant groups in Western Europe such as the Normans began to influence the vernacular and in turn evolve the language. An example of Middle English (technically Late Middle English) is Chaucer's Canterbury Tales.

I personally have no trouble understanding what is being said and the speech of the different groups is meant to convey flavor and personality to regional dialects.
I feel it adds to the game.