Yes it's possible.
I have mine hx4700 both with english ROMS, and Autocomplete and Spellchecker in Portuguese.
Take a look at
http://forum.xda-developers.com/arch.../t-275902.html
and
XDADeveloperWiki - Changing_Device_Language
Pay attention that to do those changes you'll need some files from a Device with a Spanish ROM.
It's also mentioned the possibility of changing the default keyboard layout to a specific language ( In your case Castellano). In in my case, I just prefer to use Resco Keyboard PRO (They have a lot of National layouts included).
best Regards
BouchyBouchy