I?m not sure what rule of English requires, or even permits, quote marks around the last two words in that sentence. I?m no prescriptivist, so I?m happy to be shown that this falls under some generally followed pattern, but it?s no pattern I?m familiar with.
Glad to help out here. There’s a long folk tradition in American roadside signmaking of using quote marks the same way normal people use italics or exclamation points. In more refined circles ? for example, Usenet or corporate memos ? this message would be rendered thusly:
This is America. When ordering, SPEAK ENGLISH!!!
Since using quote marks for emphasis is an American tradition, it’s obvious that the cheesesteak owner is using them here in a meta-referential attempt to draw attention to the fact that his shop is in America, and thus one should not just speak English, but “speak English.” It’s actually a very sophisticated play on the modalities of American discourse and the folkways of blue collar American culture.
POSTSCRIPT: The mainstream press, of course, has deliberately ignored the subtle subtext of Joey Vento’s sign and instead portrayed him as just another ignorant nativist prick. They even mock the local idiom. Typical, isn’t it?
POSTSCRIPT 2: Yes, I’m joking. But not about the quotation mark thing. Head over to The Gallery Of “Misused” Quotation Marks for more.