django-cms: The slugs of child pages are built incorrectly and return a 404 error when a user adds a custom slug for another language of the parent page

Summary

The slugs of child pages are breaking when a user adds a custom slug for another language of the parent page, this leads to incorrect urls being displayed and the correct urls return a 404 error.

Actual behaviour

1. Create an empty website with 2 languages - English and German.
2. Create a Page with the slug parent-en in English and publish it, leave the German slug empty.
3. Create a child Page of the first page with the slug child-en in English and child-de in German
4. Confirm that the created child page has the url parent-en/child-en and the respective German url, all is fine for now.
5. Open the parent Page and add a custom German slug, specifically parent-de and publish all the unpublished pages.
6. Now open the German version of the website, and see in the menu that the link to the child page is parent-en/child-de instead of the correct parent-de/child-de url. Confirm that the expected url parent-de/child-de actually returns a 404 error.

Expected behaviour

On the 6th the url in the menu should be parent-de/child-de , and the url parent-de/child-de shouldn’t return a 404 error.

Environment

• the first one

  • Python version: 3.5.3
  • Django version: 1.11.20
  • django CMS version: 3.6.0

• the second one

  • Python version: 2.7.6
  • Django version: 1.9.5
  • django CMS version: 3.5.1

The issue doesn’t appear to be present in django-cms 3.4.5 though.