Hi All ~ There has been interest off and on for a Spanish translation of How to Brew and other Brewers Publications titles but nothing that has come to fruition. The rights are indeed available for a Spanish version. Currently we have Designing Great Beers translated into Korean and shortly into Italian and How to Brew will be coming out in Russian soon. There is also growing interest in Brazil to have Portuguese versions. So don't rule this idea out but it's a matter of being matched with a motivated Spanish language publisher to facilitate.
Cheers~
Kristi