A side-effect of the latest IA implementation is that landing pages that haven't been localized are now blank instead of showing English content. The solution is to "translate" the quick and explore sections of those landing pages.
We're only concerned about /home, /mobile, /sync and /firefox-home because those are the only ones reachable from the old navigation.
Translate a landing page
Open the English "quick" section.
Click on Translate Article
Select a language to translate the page to
Copy and paste the English title into the title field
Hit tab to auto-fill in the correct slug
Click the green submit button and enter a revision comment
Click the review link in the article's history view