[FIX] mail: translate assignation msg in recipient lang

Before this commit, when assigning a user onto a record,
through the field user_id, the email was sent
in the current user's language

After this commit, it is sent in the partner's language

OPW 2060898

closes odoo/odoo#38142

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>

addons/mail/models/mail_thread.py

@@ -2870,7 +2870,9 @@ class MailThread(models.AbstractModel):
         for pid, sids, template in res:
             new_partners.setdefault(pid, sids)
             if template:
-                notify_data.setdefault(template, list()).append(pid)
+                partner = self.env['res.partner'].browse(pid)
+                lang = partner.lang if partner else None
+                notify_data.setdefault((template, lang), list()).append(pid)
 
         self.env['mail.followers']._insert_followers(
             self._name, self.ids,
@@ -2879,8 +2881,8 @@ class MailThread(models.AbstractModel):
             check_existing=True, existing_policy='skip')
 
         # notify people from auto subscription, for example like assignation
-        for template, pids in notify_data.items():
-            self._message_auto_subscribe_notify(pids, template)
+        for (template, lang), pids in notify_data.items():
+            self.with_context(lang=lang)._message_auto_subscribe_notify(pids, template)
 
         return True
 

addons/test_mail/tests/test_performance.py

@@ -200,7 +200,7 @@ class TestAdvMailPerformance(BaseMailPerformance):
     def test_message_assignation_email(self):
         self.user_test.write({'notification_type': 'email'})
         record = self.env['mail.test.track'].create({'name': 'Test'})
-        with self.assertQueryCount(__system__=39, emp=40):
+        with self.assertQueryCount(__system__=40, emp=41):
             record.write({
                 'user_id': self.user_test.id,
             })