Let the time format be language dependent

The 12/24 time format is controlled by a define.
While this define can be overridden in `site/config.php`, it would be better to make it language-dependent.
This leads to two improvements:

1. Webmasters don't have to configure the time format in their `site/config.php`: it's already defined in the localization file (.po/.po).

2. Switching between locales with different 12/24 time formats, automatically keeps the time format consistent with the current locale