util: Use (thread-safe) uselocale()

util: Use (thread-safe) uselocale()

Our helper method to translate a string according to the LC_TIME
setting uses the no-thread-safe setlocale(). While that's not an
issue in practice given that all JS code shares a single thread,
I was pointed to a better alternative when adopting the same
handling in gnome-desktop's wallclock[0].

[0] https://gitlab.gnome.org/GNOME/gnome-desktop/merge_requests/17

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/292