Look into System Preferences. Open "Text to Speech", then "System Voice", then "Customize", and download spanish voices. There are male and female Spanish and Mexican voices available.
So what is it: 10.8 or Leopard? 10.8 = Mountain Lion. 10.6 = Leopard. In 10.7 (Snow Leopard) Apple introduced high quality voices, each taking a few hundred megabyte, but significantly better than the ones in 10.6, and the ability to download about 30 more voices in all different languages.
One more reason to upgrade to Snow Leopard or Mountain Lion.
I really apologize, I meant 10.5.8 not 10.8, but you really gave me one more good reason to upgrade, I'm waiting for the next version, I wonder when it will be released, please post here if you have any idea ?