I agree with that you should be allowed to post the sign you want in the language you want.
The business owner knows his clientele. If english-only patrons don't feel welcome, they won't come, and his business suffers. The loss is his. But it should remain his choice.
I similarly wouldn't agree with obliging anyone to post signs in spanish. If there is a significant spanish-only population in your area, its probably in your interest to make your business spanish-friendly, but it should always be the business-owner's call.