If you speak three languages, you're trilingual.
If you speak two languages, you're bilingual.
If you speak one language, you're an American
There are two sides in this conversation: The first is that somebody said something insensitive and somebody wanted to correct them. The second is that they wanted to conduct it in another language.
While spanish is certainly a common "second language" in Texas, there are a number of cultural and historical reasons why isn't not an official second language. I would suggest that it is a harmless matter to clearly add a new forum rule that the forum is English-only. That eliminates the overall concern.
Personally, I am relatively conversant in French and fluent in Esperanto (a vestige of my overly-liberal youth
), but I don't think I would post anything in either language in an attempt to drive home a point.