However, whilst this is fine for existing mail databases, I don’t know what the resolution is for newly created users. I don’t think it is possible to set this in the mail template.
I think you might find that this controlled by a browser and/or client OS setting rather than the server setting.
You don’t say what browser you’re using, but I’ll assume it’s Firefox. If so, try checking how your Firefox Language is set. My guess is that it’s defaulting to the U.S., and hence you get MM/DD/YYYY for your dates.
Here’s some info on how change the setting for various browsers, although I’m not sure how up to date all this is:
Make sure you restart your browser after making the change.