Issue:
------
After registering for an event, you can download the '.ics' file.
This file is not translated into the current user's language.

Solution:
---------
Use the default user language and, if public,
use the language selected on the website.

A user may wish to have his calendar in his preferred language
by using the English website.

opw-3193663

closes odoo/odoo#134465

Signed-off-by: Thomas Lefebvre (thle) <thle@odoo.com>