An embarrassing idiot strikes again. Just say “thanks, I didn’t understand.”
They would probably have given him the special if he hadn’t pitched a fit and put everybody’s backs up. I’ve seen this happen in Chinese restaurants, where they have posted their specials policy in English but it’s not very clear.
The embarrassing idiot is the one who puts up their sign in a foreign babble and then won’t go ahead and make it right when the inevitable happens and an American doesn’t understand.
The morons could have apologized, and given him a taco for free. They woulda had a customer for life. Instead they demand everyone learn Mexican.