Kind of an authoritative support for Dave's answer (*1) as this was a question I asked Mr. Baer on a visit in 2008. I haven't heard that term before either.
But why call it "English"?
As Dave said, it's a term taken from billiard and about giving it a 'spin'. The intention was to make the game more about skills. He assumed that a straight deflection would be way too boring.
In the center of the left knob is yet another knob which controls the "ENGLISH" of the Ball Spot. I think "ENGLISH" refers to something in the real-world game of billiards that governs how a ball's trajectory curves due to its spin. This "ENGLISH" control allows a player to control the trajectory of the Ball Spot after deflecting it with the Player Spot. The Odyssey Manual always capitalizes the word "ENGLISH" so forgive me if you think I'm shouting.
from an Gamasutra feature (*2)
Regarding the capitalization, the manual does mark all UI elements that way - but, unlike the article mentions, not always. Regarding the use of English the manual states:
English. The English control affects only the ball. When the
ball is travelling from left to right, only the Player 1 English
control will affect the ball. When the ball is travelling from
right to left, only the Player 2 English control will alfect the
Ball. The English control will cause the Ball to move upward
or downward, depending upon the rotation ol the control
knob. The further the knob is turned, the more the Ball will
deflect. For your ease in determining the approximate
centering of this English control, a "raised" marking is
provided on the knob. This marking should be in the
upright center position of the knob when you return the
Ball to the screen
From the manual p.7
What does a language (or a nationality) have to do with centering a ball ?
It's not just centering, it as well defines how the game starts and how the ball is deflected. For the term itself, no idea, but then again, many things got wierd names that only make sense within a certain setting.
Jim Nelson found some quite helpful answers about the usage of this meaning of 'English' on the English Language & Usage SE site:
'Why is putting some spin on a ball described in some circles as giving it some "English"?'
And, especially, why label it that way to the user ? Is it something that made sense to the general public in 1972 ?
For sure more than any newly invented term would have. Keep in mind, back then it was still common to play real games with real people in real pubs. It's always better to take an even marginally known term then to invent something. Just think, noone ever pressed a button on screen - at least not before touch screens became a thing. Still was named that way :)
*1 - Please mark his answer.
*2 - Great article about Pong history with many details, worth reading.