Ensure all pages notify users when no translation can be found

[rschristian]

While working on #899, I've noticed that we don't show the "Missing translation" warning banner on every page. The tutorial and home page do not provide any warning, only remain in English even after the user has selected an alternative.

If a user were to say pick Korean on the home page, they might be a bit surprised to see no change in the content whatsoever (as we don't yet have a Korean translation). We should fix this.