As I used FusionInvoice until recently, I kind of made the first spanish translation and keept improving it until now. There are some concepts that really matters for spanish (Spain) user that goes different for Latin American users, and the tool to translate does not have this kind of localizations.
So, I updated the translation and provide the file for whoever wants to use it. Feel free to copy, improve or do anything you want.
I colaborated with project Gnome before, so most of conventions are from its official guide.