Things pertaining to the land or culture of England are referred to as being English. This includes the people of England as well as the language spoken there, which was brought to the American colonies by the English explorers.
The English language finds its origins in the West Germanic tribes that inhabited England in the 5th Century: the Saxons, the Jutes, and the Angles, the latter of which provided the name English. Mix in Romance languages such as the Latin-based Norman French and some old Norse, and you have the smorgasbord that is our modern English. Note that the english or "spin" you put on a billiard or bowling ball seems to have evolved separately, from the French anglé, meaning "angle."