In researching other slug bugs, I found something very bad: the incorrect translation slug is sent to the translation edit page. To duplicate:
1. Create an en-US document with a slug "html"
2. Translate to French with a slug of "somethingelse"
3. Click the edit button for the French document.
4. Click "save changes"
5. You'll see that the slug says "html", which is the slug of the root/parent document, *not* the French translation
My research shows that this never quite worked correctly. If I pass the document_slug as the revision.slug, the translate page 404s.
CC'ing other devs on this because it's a fairly large issue.
A few other things I'll point out:
1. Redirects are being created properly, so this isn't the end of the world
2. This is probably causing the occasional "slug is taken" warnings.
Commit pushed to master at https://github.com/mozilla/kuma
fix bug 809617 - Prevent document slug override
verified fixed translated the https://developer.allizom.org/en-US/docs/HTML/HTML5 to French
https://developer.allizom.org/fr/docs/HTML/HTML5testFrench and the new slug was saved