Geno's, the legendary cheesesteak shop in my hometown of Philadelphia, is now being sued for a sign demanding that patrons order in English. Read the hundreds of comments, and you can see that Philly's bursting with staunch defenders of the language of Dickens and Shakespeare, not to mention Rizzo.
I've lived in foreign countries and struggled with foreign languages. It's a very difficult moment to order something in your native language when you're not sure it will be understood, or to piece together a sentence in a language you barely know how to pronounce. You take time to pick places that look hospitable and friendly. So unless Geno's hires a lot of Spanish speakers to man its counters, I don't think'll face a rush of Mexicans ordering their sandwiches in Spanish. Mexicans don't need to read a sign to know where they're not welcome.