Well, that's no bueno! Even with Spanish Spanish, you should be able to order a taco and a cerveza por favor in Mexico.

Often textbook Español is just basics anyway; the dialects are different everywhere so you might be fine anyway.

You should be fine, chill out. When I went to Spain the more Mexican Spanish I knew worked fine. So the other way around will be fine also. It's just the way some things are said and that is nothing to worry about.

Hey, at least you got close. I studied Latin in high school for four years to travel to Latin America. When I got there, no one understood when I said, "Omnia Gallia in tres partes divisa est." And they thought my toga was "muy loco," whatever that means!

It can't be all that different.

Spain Spanish uses the vosotros conjugation, and pronunciation and slang varies by region, but it's still the same language. I'd be more worried about how textbook Spanish often doesn't teach the most common words for things (like it teaches "el coche" instead of "el caro").

It'll be fine. They're about as different as American English vs British English. There are definitely some differences, but you should be able to communicate. If someone says something you don't understand, try asking them to word it a different way.