Oh, now i recall, you need a program named UltraEdit and PoEdit.
Open the .po file in UE.
then
File > Convert > UTF-8 to Ascii
Change where it says UTF-8 for iso-8859-1,save, open with PoEdit,check the setting, it must say iso-8859-1.
Save and upload, that must be an iso file in spanish.
Oh, btw, i used the es_Es translation from es.www.ads-software.com