[FIX] website_slides: translate course invite into recipients' language

Problem:
when adding attendees to a course, the sent email is not translated into
the recipient's language.

Fix:
Upon sending, translate the email on a per recipient basis,
but **only when** the final message to sent is exactly the same
as the template. (i.e. the body was not edited from the dialog box)

This is a compromise to permit emails being translated while not
breaking the following 2 current features:
* there can be many recipients (with different languages)
* the body is pre-filled with a template but can be edited further
  by the user directly from the dialog box.

opw-3862369

addons/website_slides/tests/test_attendee.py

@@ -514,3 +514,43 @@ def test_members_invitation_expiration(self):
         self.assertFalse(channel_partner_portal.exists(), 'Expired invitations should be removed, even if archived')
         self.assertTrue(self.channel_partner_emp.exists(), 'Memberships with progress should never be removed, even archived.')
         self.assertFalse(self.channel_partner_no_user.exists(), 'No Last Invitation Date is considered as expired for invited members')
+
+    def test_invite_email_translation(self):
+        "Make sure that invitation emails are translated if unchanged when adding attendees to a course"
+        self.env['res.lang']._activate_lang('fr_FR')
+        jean = self.env['res.partner'].create({'name': 'Jean', 'lang': 'fr_FR'})
+
+        template = self.env['mail.template'].create({
+            'subject': 'Hello',
+            'body_html': 'en',
+        })
+        template.lang = '{{ object.partner_id.lang }}'
+        template.render_model = 'slide.channel.partner'
+
+        template.with_context(lang='fr_FR').subject = 'Bonjour'
+        template.with_context(lang='fr_FR').body_html = 'fr'
+
+        channel = self.env['slide.channel'].create({'name': 'Test Course'})
+        slide_channel_jean = self.env['slide.channel.partner'].create({
+            'channel_id': channel.id,
+            'partner_id': jean.id,
+        })
+
+        wizard = self.env['slide.channel.invite'].create({
+            'send_email': True,
+            'partner_ids': [jean.id],
+            'channel_id': channel.id,
+            'template_id': template.id,
+        })
+
+        mail_vals = wizard._prepare_mail_values(slide_channel_jean)
+        self.assertEqual(
+            mail_vals['body_html'],
+            'fr',
+            "Mail body should have been translated into recipient's language"
+        )
+        self.assertEqual(
+            mail_vals['subject'],
+            'Bonjour',
+            "Mail subject should have been translated into recipient's language"
+        )

addons/website_slides/wizard/slide_channel_invite.py

@@ -86,8 +86,13 @@ def action_invite(self):
 
     def _prepare_mail_values(self, slide_channel_partner):
         """ Create mail specific for recipient """
-        subject = self._render_field('subject', slide_channel_partner.ids)[slide_channel_partner.id]
-        body = self._render_field('body', slide_channel_partner.ids)[slide_channel_partner.id]
+        set_lang, lang_opt = {}, {}
+        subject_same_as_template = (self.template_id and self.subject == self.template_id.subject)
+        if self.body_has_template_value and subject_same_as_template:
+            lang = self._render_lang(slide_channel_partner.ids)[slide_channel_partner.id]
+            set_lang, lang_opt = {'set_lang': lang}, {'lang': lang}
+        subject = self._render_field('subject', slide_channel_partner.ids, **set_lang)[slide_channel_partner.id]
+        body = self._render_field('body', slide_channel_partner.ids, **set_lang)[slide_channel_partner.id]
         # post the message
         mail_values = {
             'attachment_ids': [(4, att.id) for att in self.attachment_ids],
@@ -112,7 +117,7 @@ def _prepare_mail_values(self, slide_channel_partner):
                 'company': self.env.company,
                 'signature': self.channel_id.user_id.signature,
             }
-            body = self.env['ir.qweb']._render(email_layout_xmlid, template_ctx, engine='ir.qweb', minimal_qcontext=True, raise_if_not_found=False)
+            body = self.env['ir.qweb']._render(email_layout_xmlid, template_ctx, engine='ir.qweb', minimal_qcontext=True, raise_if_not_found=False, **lang_opt)
             if body:
                 mail_values['body_html'] = self.env['mail.render.mixin']._replace_local_links(body)
             else: