Improve 12/24 clock detection

Improve 12/24 clock detection

The current check looks at whether there was some am/pm suffix in the
localized time; however in some cases (locales) that does not seem to
work. this version instead checks whether the localized versions of 05:00 and
17:00 start with the same two digits; if not, it's probably a 24h clock.

https://bugzilla.gnome.org/show_bug.cgi?id=603488