The history of English begins with the ancestorship of the Germanic language which in turn is a combination of languages from the European branch of languages which dates back to the first millenium before Christ (BC). Technically speaking at this point, the history of English could and should be considered as starting with the European family of language and not the Germanic. In the end the Germanic people later gave rise to the English language of the Angles, Saxons, Frisians and Jutes. This in turn was considered as proto English. The age of Celtic English came from the British celts.
This language shift started about the 5th century. Instead of using borrowed words and incorporating them into their own words and slang, the British Celts made bilingualism more popular. This dialect encompasses many geographic locations such as Wales, Cornwall, Ireland and Scotland. Even with this slight shift in the language we all now speak, the influence of the original celtic language has very little to do with the English language though this is still considered as part of the Old English. Norse Vikings, Beowulf and the Norman conquest are examples of the time period activity and language.
This language adaption lasted only about 700 years, from the Anglo migrations in the 5th century to shortly after the Norman Conquest in the 11th century. Middle English started shortly after the Norman conquest and skated through the days of The Canterbury Tales and Chaucer. By this time the English language was quite diverse and was furthered by the invention of the printing press where the English used during this period was hit fairly heavy. Even with classics and the printing press, this early middle English was only used for approximately 580 years, to about the year 1650.
At this point the language shifted again to what is known as Early Modern English. The history of the great vowel shift. The great vowel shift changed the face of English entirely. The vowel shift changed not only the way words were spoken but also the sound of the words. The vowel shift added in vowels and changed the face of how words were written. Technically the vowell shift was first introduced during the same time as the early modern English, mainly it was the name change of modern to the vowel shift that designated the difference of names but in the end the vowell now known as simply Modern English, which is what we speak today.
This shift while even though it was happening during the same time as the middle English, it also historically gave the separation between the middle and modern language. Linguistic historians are still puzzled not only by the speed in which the vowel shift happened but also by how long it has remained as we have been using the vowel shift dialect of the English language for well over 350 years with very minimal changes in sight. Their theories of the mass amount of people, speaking different languages and the need to find a standard language may have been the precipice for which the vowel shift happened.
Knowing that the face of the English language has changed and has a broad geographic background it only stands to reason that there will be a future English which will become complex and linguistically speaking may not even sound likt the English we are used to. Technically we see this shift very slowly happening by the addition of new words and slang complemented with different pronunciations and like those who speak spanish and English and in this, this is called spanglish. Our English very well may become convoluted altogether with a totally different language.