In fact if the WTS did their research properly they’d realise that it dates back to the days of the bubonic plague. One of the first signs that you had the plague was sneezing so people would say “bless you” wishing/hoping that you wouldn’t come down with the plague.
As Scratchme says, in Spanish we say “Salud” or “Jesús”. I personally always say “salud” as a sign of respect and courtesy. It’s automatic. I also say it when having a drink with a non-JW or another PIMO JW (in this context, salud is translated as “cheers”.)