I know I can put top and backspin on the ball in Wii Sports Tennis, but the computer seems able to english (left and right spins) on the ball. Can humans do it, or is it something only the computer can do?
You're talking about sidespin, and yes, humans supposedly can put sidespin on the ball. Here's an example that demonstrates how to do it: instead of following through with your wrist, don't move your wrist at all, just move your arm.
I was unable to get this to work - all I got was straight shots, no spin at all - but there might be something about the video I'm not understanding.
It's actually not that difficult if you know what you're doing. The trick is that normally when playing WST, you will rotate your arm around your body, and the Wii Remote will be rotating with your body as the pivot point.
Rather than rotating the Wii Remote, you want to slide it to the side, so that the remote doesn't really rotate much at all. Think about it as pushing the Wii Remote diagonally (forward and across your body), without rotating it at all. Thus, the remote will still face left-to-right (or right-to-left) from the beginning to the end of the swing.
I'm not sure how to better explain it, but if you watch @Dave DuPlantis's video, you should be able to see that in his standard swing, the tip of the Wii Remote will be pointing a different direction at any given point in the swing. (Directly away from the center of your body) When slicing the ball, the Wii Remote will always wind up pointing approximately the same direction, as you slide the "raquet" across the front of your body, instead of rotating it around your body.
P.S.-It may or may not be worth noting, but I always had a better slice when I would actually angle the remote not directly left-to-right, but slightly backwards, so that the IR sensor was pointing slightly behind me, when swinging. I don't know if that will make a difference to you, but it works well for me.