Around the second century BC, the Germanic language split into three distinct sub-groups; East Germanic was spoken by people who migrated back to southeastern Europe (East Germanic is not spoken today and the only written material that has survived is Gothic), North Germanic evolved into the modern Scandinavian languages of Swedish, Danish, Norwegian and Icelandic (Finish is not an Indo-European language but is related to Estonian), and the West Germanic sub-group which is the ancestor of modern German, Dutch, Flemish, Frisian, and English
(Literature: The British Edition p.8).
The history of the English language can be traced back to the arrival of three West Germanic tribes to the British Isles during the 5th and 6th Century AD. Angles, Saxons and Jutes crossed the North Sea from what we know today as Denmark and Northern Germany. The people of Britain until this time spoke a Celtic language, but the invaders quickly pushed the original Celtic speaking inhabitance out of what is now England and into Wales, Scotland, Cornwall and Ireland. These Celtic languages survive today in the Gaelic languages of Scotland, Wales and Ireland. .
During the next few centuries four dialects of English developed: Norhhumbrian in Northumbria (north of the Humber), Mercian in the Kingdom of Mercia, West Saxon in the Kingdom of Wessex and Kentish in Kent. (Lecture Notes N. Ordenova) During the 7th and 8th Centuries, the Northhumbrian language and culture dominated Britain, but the Viking invasion of the 9th Century brought an end to this domination along with the total destruction of the Mercian culture and language. The only culture that remained unharmed by the Viking invasion was that of Wessex, which continued as an independent kingdom. Therefore, by the 10th Century the official language of Britain had become the West Saxon dialect. It was also in the West Saxon kingdom that a written language first flourished.