I searched on Google but I didn't find any info. I just noticed that previous spanish language packs are also facing this issue.
The point is that this happens when sending automatic PMS to users -i.e. related to connections-, notification mails and popups.
Switching to non UTF-8 code is not an option. It is well known that joomla is taking some efforts to allign all codificatioins. The forward path is to remove some htmlentities on the core code, but I would prefer to mantain it "virgin".