Read all localized urls in content

Feature needed for:

* Language-Switcher
* HREF-LANG attributes in `sitemap.xml`
* HREF-LANG tags in html pages

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | ???
| BC Breaks     | ???
| Doc           | ???