1) SolidWorks does not auto-translate existing FeatureManager tree features. So new features would be created in Spanish, but the existing ones already existing in SolidWorks documents will be left in English.
2) Relating to #1, if you already have document templates created in English, then the default plane names, origins, etc. will be in English in those templates. You will have to back up those templates somewhere, remove them from their current location so SolidWorks does not find them and then it will automatically create new templates with the Spanish language.
I hope this helps,