I don't see how it could be discrimatory, they have "We reserve the right to refuse service to anyone" signs around here. I hope this catches on I need to get some signs for the garage, or maybe a tee-shirt.
Personally I dont see the need for the english only sign. If you come in to my establishment and order up services in any language other than english I am gonna stand there and stare at you, cuz I have NO IDEA what you are wanting. Seems like that would be a bit of a stalemate.
Truth be told I might be able to speak a bit of french and a bit of dutch if hard pressed