from what a quick google tells me, it has only been translated into Russian and German. It was supposed to go into Italian (they leased rights for a year) but I never found the completed translation.
Speaking as a professional translator, a whole book in a specialty niche like this takes me tens of thousands of dollars worth of time. When I quote US16,000 for a book, the client often runs away screaming. They then have to find some kind-hearted yet capable unicorn who is willing to do it as a labor of love (and who has no bills to pay). Which is why only books that are real money-makers are translated (since they have an agency that is throwing the cash around).
There is also the fact that Bill Mollison can do whatever he likes with his books. The rights belong to him, and if I were to translate into Spanish just for fun, I need his permission (and/or I need to pay him) to use his intellectual property.
I know you are talking about a specific book but I stumbled across some biointensive materials in many languages, including Spanish, recently and the Mexican family might find some useful.
http://www.growbiointensive.org/Self_Teaching.html