[FIX] mail: template, format date to customer language

Up to now, there was no possibility to format a date
within a specific language, such as the customer
language. The date was always displayed with the format
YYYY-mm-dd.
(There was for the datetime, thanks to format_tz, but
not for a simple date without time)

This revision adds this possibility, but doesn't change
the according email templates
(such as the invoice date on the invoice email template)
as it would require to update all translations, in
the .po files and on transifex.

If a user absolutely want this, he has to add
the call to `format_date` himself, manually.

opw-704260