🔨(backend) email invitation in invited user's language

- language for invitation emails => language saved on the invited user
- if invited user does not exist yet => language of the sending user
- if for some reason no sending user => system default language

src/backend/core/api/viewsets.py

@@ -1167,13 +1167,14 @@ class DocumentAccessViewSet(
     def perform_create(self, serializer):
         """Add a new access to the document and send an email to the new added user."""
         access = serializer.save()
-        language = self.request.headers.get("Content-Language", "en-us")
 
         access.document.send_invitation_email(
             access.user.email,
             access.role,
             self.request.user,
-            language,
+            access.user.language
+            or self.request.user.language
+            or settings.LANGUAGE_CODE,
         )
 
     def perform_update(self, serializer):
@@ -1399,10 +1400,11 @@ class InvitationViewset(
         """Save invitation to a document then send an email to the invited user."""
         invitation = serializer.save()
 
-        language = self.request.headers.get("Content-Language", "en-us")
-
         invitation.document.send_invitation_email(
-            invitation.email, invitation.role, self.request.user, language
+            invitation.email,
+            invitation.role,
+            self.request.user,
+            self.request.user.language or settings.LANGUAGE_CODE,
         )