The English language was started when three Germanic tribes – the Angles, the Saxons and the Jutes – invaded Britain during the 5th century. At the time of the invasion, Celtic was the dominate language of Britain.
In Britain, the language spoken by the Germanic tribes developed into Old English. English speakers of today would have difficulty understanding Old English; however, approximately half of the most common words used in Modern English have roots in Old English. Old English was common until the invasion of William the Conqueror in 1066.
The Normans brought with them a version of French to Britain which became the language of the Royal Court and the ruling class. During this time, the lower classes spoke English and the upper classes spoke French. By the 14th century, English was again the common language for all speakers, but it had changed significantly from Old English into what we now call Middle English. The lower classes simplified the language, and the upper classes added many French words.
With world travel and the industrial revolution, English in Britain developed into Modern English which is the version spoken today.