For the beginning, use the spanish translations in the english version. Two #defines at the front of the .fr should get you far.
Next, download the spanish demo/release installer. Eventually install it to a different machine (boot disk, VM) and move its folder to your development machine, rather than risk that the installer screws up your other installations. Keep that other box around because update installers will fail on anything unexpected.
Finally, there used to be some tricks to switch the language of the debug build. On the Mac you had to change the version resource, I think later on that turned to the info.plist . Windows required regedt32 on some numeric value. I haven't tried either of these for CS3.
Dirk