I had a co-worker, a staunch Democrat, non-racist, liberal, Westsider who always used to joke about it: "I can speak Spanish, listen, 'Open-o the door-o.' You just add an 'o'!" I don't think telling jokes makes a person a racist, but it kind of bugged me. I mean, did he think Spanish was some kind of derivative language?
At some point my mom and I were planning to go to Italy, and I bought "Learn Italian" CD's to play in the car. My mom, who doesn't speak English, would listen and repeat some of the phrases, then one day, after the lesson involving trains, she observed, "You know, Italian is almost identical to Spanish, but with an 'o' added on." I almost crashed the car, I was laughing so hard. So I forgave my co-worker -- I mean, seriously? What could I say, after my own mother said something like that? It still makes me laugh.
Speaking of Italy ... the reason that so many Mexicans refer to Protestants as "Cristianos" is that it's how they evangelized in Mexico. They claimed that Catholics weren't Christian, and distinguished themselves by claiming Christianity as their exclusive description. By the way, Catholics in Mexico (at any rate, the ones I know) don't refer to protestant groups as "churches"; we call them "sects."