Bug 1333197 was partly caused by missing support for system time zone change observers on all supported platforms . We've added a workaround resp. requested a specification change for the ECMAScript Intl API, so the missing support doesn't lead to Talos regressions like bug 1314354, but ultimately it makes sense to get rid of this call , and instead use OS events to get notified when the system time zone was changed.  bug 1333197, comment #1, bug 1333197, comment #11  http://searchfox.org/mozilla-central/rev/31b6089ce26fa76459642765115605d50a6c67b4/js/src/jscompartment.cpp#138-143
Forgot if I asked you already, but since we now have intl/locale/OSPreferences which is an intl related OS-specific code hooks that will have all the event listeners for reacting to system language changes, region preference changes etc. Would it make sense to locate it there?
I don't mind where this functionality is implemented, as long as it is available. ;-)
https://github.com/whatwg/html/pull/3047 will probably also require this functionality.