How can I teach the history of the English language through the ages from prehistoric Celtic times to the present time?

Along with other influences upon English, the Danes affected it early on. In fact, Old English began with the Angles, a people who originated in a section of Denmark still called Angeln. First, they contributed at least 900 words, such as husbondi and syster (husband and sister).  Second, the Danes added pronouns to English, filling a real linguistic need.  Most significantly, before the arrival of the Danes, Old Englsh conveyed meaning through word endings, like Latin, instead of word order.  The Danes simplified language by elminating this  use of inflection.  This process was completed when Middle English evolved.

With the Norman Invasion of 1066, the language of the nobility became French after William the Conqueror became king.  He confiscated the lands of the Anglo Saxon nobility, allowing the Normans to control government at all levels.  Consequently French became the official language.  Medieval literature was in French, such as the tales of King Arthur.  To this day over half of the words in English are of French derivation. 

With reference to Geoffrey Chaucer, he was in touch with French poets and translated poetry and wrote his own in French and sought to reproduce both the matter and style of Fench poetry in his own.  But, he decided to use the English of his day for his Canterbury Tales.