It's one of the Red Court nobles who uses the word, which makes sense given some of them would have been conquered by (or perhaps themselves were) Spanish invaders. It irritates Harry a bit irrationally as he has a foolish sense of ownership of the word, even though it's quite likely there would be other magic users that use Spanish as their magic words.
I won't get into how using a live-language would have so many problems (not to say the least if he actually learned Spanish, it should in theory diminish the protection provided by those words). Which seems to be why a lot of wizards use older languages (many of which are no longer in use) e.g. Latin, Ancient Egyptian, Sumerian, etc.
The Grey Council had indeed arrived by this point.