datetime: Ensure that translated strings for timezones are UTF-8

This was fixed in commit 2c4bba74e07405205b22c684b3822d13aa429ea7 but
then got lost in a code refactor in commit
3686cf7eb8528b19597945e2c19ba0f619ef42f6.

https://bugzilla.gnome.org/show_bug.cgi?id=706350