It's not racist to display the sticker, but it IS rather redundant. It's virtually meaningless. Let's say you get into a cab, driven by a chap from Somalia, who speaks very poor broken English...well, he could still put up a sign saying 'English spoken here', couldn't he?
The sign doesn't say 'Excellent English spoken here', so the intent is lost.
Personally, I'd be happier with a sign which read 'No English, or any other language spoken here'. Can't stand talkative cab drivers. I'd pay extra for silence.