In looking up the answer to this question I discovered a small bug. Currently its using the cultureinfo of the executing thread to choose which file to use but really it should be based on the site default culture. So your browser language preference is set to english so it uses the file /Data/MessageTemplates/en-US-RegistrationConfirmedEmailMessage.config
It would use a different file if your browser language is set to something different for which we have a translation.
for now you can edit that english file but you can also create one with the prefix of your preferred language because I will fix this for the next relese to make sure it uses the site language as specified in Web.config as documented at the bottom of this page in the section about forcing a specific culture.
Hope it helps,