Geno says he's never turned anyone away for not speaking English, but I don't know if he's got any staff who can understand other languages.
In any case, how likely is he to get a customer who can read English but not speak it?
*At least one news story and Wikipedia misquote the sign as saying "please speak English". I've corrected Wikipedia. Let's see if it sticks.
Addendum: I don't know how to check the Hispanic press and blogosphere to see how the sign has gone over among Hispanics. I will note that there's Pat's, another cheesesteak place, across the street, and they're both about equally busy.
Added Addendum: It turns out that the sign really does say "Please speak English", as agrumer points out--it's just that the "please" is rather hard to see. The Wikipedia article had already been changed back by the time I checked on it.