After some research, I found some sources that indicate that it comes from the expression **body english**, which indicates the body gestures and movements that go with the speaking:

> [...] Oddly enough, the origins of "English" don't appear in the Oxford English Dictionary, even though the word is right there on the cover. False advertising, sez I. *However, other sources are pretty much in agreement. "English" comes from "body English," the contortions a thrower/roller/hitter goes through after the ball has left the hand/club/cue. These motions are called body English because they relate to the physical gestures we employ when we speak. Which is differentiated from "body language," emotions communicated through posture rather than gesture.*

> So, while "body English" is what we do after the ball is in motion, the term "English" is reserved to describe motion actually put on the ball by its spin. What do the English call "English"? "Side." As in "Don't hit it in the center, hit in on the ..." Sometimes the English make sense, even if "English" doesn't.

><sup> **Emphasis mine. Taken from [here][1].**</sup>

There's similar stuff written in [this forum][2]:

> ENGLISH: As you note in your question, "english" (always lower case) is spin put on a ball, usually but not always a billiard ball (baseball, tennis and golf balls can also be "englished"). The British call the same effect "side," because it is accomplished by striking the ball slightly off-center, thus imparting the spin. According to billiard experts, this spin allows the ball to do all sorts of remarkable things, such as curve, hide behind other balls, and disappear entirely when really needed. If I had paid attention back in high school physics while they were discussing vector momentum, I'd be able to explain how all this works, but I wasn't and can't, so I suppose we'll let the scientific discussion go at that. 

>*As to why they call the spin "english," the answer turns out to be surprisingly simple. It is derived from the actions that the player makes to cause the ball to spin -- the extra gestures, physical effort and "oomph" we know as "body english." We call those contortions "body english," incidentally, because such physical gestures (waving your hands, hopping up and down, etc.) are sometimes used to boost to the expressive effect of our spoken English.* [...]

> <sup> **Emphasis mine again.**</sup>

Hope it helps. A little note: There is more in the forum stuff, I didn't report it for space-allocation reasons. I'm aware these might not be the most official or reliable sources, but there is nothing on dictionaries, which makes me think it's not something really that clear, yet. If anyone disagrees or wants to comment, feel free to.
  [1]: http://www.straightdope.com/columns/read/1634/whats-the-origin-of-english-the-kind-you-put-on-a-ball
  [2]: http://www.wordwizard.com/phpbb3/viewtopic.php?f=7&t=19922