[IMP] event: add a lang field on event.

In order to allow all communication to be sent in
a chosen language, a 'lang' field is added on the
event.event model. If set (amongst the installed languages)
this language will be used to translate email templates
instead of the partner's one (which is still the default one)

The field is added on the event form view.

Task-3357099

closes odoo/odoo#127540

Signed-off-by: Stéphane Debauche (std) <std@odoo.com>

addons/event/data/mail_template_data.xml

@@ -23,7 +23,7 @@
     </t>
 </div></field>
             <field name="report_template_ids" eval="[(4, ref('event.action_report_event_registration_foldable_badge'))]"/>
-            <field name="lang">{{ object.partner_id.lang }}</field>
+            <field name="lang">{{ object.event_id.lang or object.partner_id.lang }}</field>
             <field name="auto_delete" eval="True"/>
         </record>
 
@@ -257,7 +257,7 @@
 </table>
             </field>
             <field name="report_template_ids" eval="[(4, ref('event.action_report_event_registration_full_page_ticket'))]"/>
-            <field name="lang">{{ object.partner_id.lang }}</field>
+            <field name="lang">{{ object.event_id.lang or object.partner_id.lang }}</field>
         </record>
 
         <record id="event_reminder" model="mail.template">
@@ -477,7 +477,7 @@
 </td></tr>
 </table>
             </field>
-            <field name="lang">{{ object.partner_id.lang }}</field>
+            <field name="lang">{{ object.event_id.lang or object.partner_id.lang }}</field>
         </record>
 
     </data>