gsettingsschema: fix l10n=time attribute

It's supposed to be possible to translate settings values using LC_TIME rather than LC_MESSAGES to determine which translation to use, but Sebastian Keller noticed that it's not working properly. I've implemented his proposed solution, which is to actually temporarily change LC_MESSAGES to match LC_TIME for just as long as necessary to force gettext to use the desired message catalog.

Fixes #2575 (closed)