[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/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: