Facetious: When it gets called English.
Okay, the answer to your question is simple: The boundary between a language and a creole is a fuzzy one. English, born at a crossroads of sorts, is a good example of this.
English is the product of a series of serious culture clashes. Germanic tribes migrated to the islands in the fifth century AD, speaking something that would have sounded more like German than anything else. Later, the Vikings began their campaigns, spreading Old Norse (a language very close to what is still spoken in Iceland) to the benighted isles. Such is the origin of Old English. In 1066, French-speaking Normans from, well, Normandy (that place in France D-Day later happened) conquered the Anglo-Saxons at the Battle of Hastings. This brought French and, through it, Latin, to the mix. Norman French became the language of the upper classes, Anglo-Saxon (aka Old English) remained the tongue of the poor, and they mixed to create Middle English, the language of the great poet Chaucer. Then, not long before Shakespeare’s time (1600s), the Great Vowel Shift occurred, something that changed how we pronounced all of our vowels and really changed our spelling. Modern English (the tongue, not the band :)) was born in time for Shakespeare to use an early version of it, commonly called Early Modern English.
That was a very compressed history of the English language. I might have gotten some details wrong, but I think the bulk is correct, as I have seen that basic story told in numerous places. Anyway, this mish-mash of tongues created a very unique European language: Huge vocabulary, no grammatical genders, and a seriously screwed-up orthography (aka spelling). The vocabulary comes from repeated borrowings from numerous languages scattered around the globe. Most of our Latin comes to us through Norman French, though, a relic of a battle lost long ago. The lack of genders comes from the fact that the biggest impediment to understanding across different dialects of Old English was how genders were handled, so we dropped them. The orthography comes from numerous places: The Great Vowel Shift; borrowings where we mangled the spelling, pronunciation, or both; or irregularities in the languages English itself evolved from.
To summarize, English was once a creole of Germanic and Old Norse, Old English and Norman French, and then it spread around the world to pick up words and ideas from the most diverse places. It’s considered a language simply because it has been around a long, long time and has many thousands of speakers.