[FIX] mail_show_follower: Use company from the document to render messages correctly + proper lang handle

In a multi-company environment, if one company uses the
show_followers_partner_format by partner_name and another company
uses partner_email, a user with permissions in both companies may
experience issues. If a document belongs to Company A, but the current
company is set to Company B, the message format is taken from the
current company (Company B) instead of the document’s company
(Company A). This fix ensures that the message format is rendered based
on the document's company.

It also fixes the lang handling for cases with False values.

mail_show_follower/models/mail_mail.py

@@ -72,14 +72,16 @@ class MailMail(models.Model):
             show_in_cc_recipients = recipients._filter_shown_in_cc(show_internal_users)
             if len(show_in_cc_recipients) <= 1:
                 continue
-
-            langs = (
-                mail.notification_ids.res_partner_id.mapped("lang")
+            lang = (
+                mail.notification_ids.res_partner_id[:1].lang
                 or mail.author_id.lang
                 or company.partner_id.lang
+                or "en_US"
             )
-            final_cc = mail.with_context(lang=langs[0])._build_cc_text(
-                show_in_cc_recipients
+            final_cc = (
+                mail.with_context(lang=lang)
+                .with_company(company)
+                ._build_cc_text(show_in_cc_recipients)
             )
             mail.body_html = final_cc + mail.body_html