[FIX] event: translate ICS file

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>

addons/event/controllers/main.py

@@ -10,10 +10,10 @@ class EventController(Controller):
 
     @route(['''/event/<model("event.event"):event>/ics'''], type='http', auth="public")
     def event_ics_file(self, event, **kwargs):
+        lang = request.context.get('lang', request.env.user.lang)
         if request.env.user._is_public():
-            frontend_lang = request.httprequest.cookies.get('frontend_lang')
-            if frontend_lang:
-                event = event.with_context(lang=frontend_lang)
+            lang = request.httprequest.cookies.get('frontend_lang')
+        event = event.with_context(lang=lang)
         files = event._get_ics_file()
         if not event.id in files:
             return NotFound()