about:tor is broken in unsupported locales in the multi-lingual Tor Browser

While testing the multi-lingual 64-bit hardened build] (downloaded 2015-10-21) I found a locale-related bug that will be relevant for Tails, but perhaps not the vanilla Tor Browser.

In Tails we run the Tor Browser without Tor Launcher (it's available as a standalone XUL application, though), and we set intl.locale.matchOS = true so the locale that the user picked in our Greeter is used. So once we use the multi-lingual Tor Browser in Tails we won't have the language selection dialog, and hence the user can end up with a locale that isn't available in that dialog.

I have found that when an unsupported locale is used, about:tor is broken:

XML Parsing Error: undefined entity Location:
jar:file:///XXX/tor-browser/Browser/TorBrowser/Data/Browser/profile.default/extensions/torbutton@torproject.org.xpi!/chrome/content/aboutTor/aboutTor.xhtml
Line Number 207, Column 3:
  <form action="&aboutTor.searchDCPost.link;" method="post">
--^

This happens, for instance, when setting these locales:

export LANG="fi_FI.UTF-8"
export LANG="fur_IT.UTF-8"
export LANG="wx_YZ.UTF-8"

I have seen issues like this before in XUL vs DTD translation, and it's pretty similar to legacy/trac#11483 (moved), so it could just be that some translation string is missing, or that we don't fallback to en-US properly.