The default value for /schemas/apps/clock_applet/prefs/format
has the comment:
<!-- Translators:
               This controls whether the GNOME panel clock should display time
in 24 hour mode
               or 12 hour mode by default. The only valid values for this are
"24-hour" and
               "12-hour". If your locale uses 24 hour time notation, translate
this to "24-hour".
               If your locale uses 12 hour time notation with am/pm, translate
this to "12-hour".

               Do NOT translate this into anything else than "24-hour" or
"12-hour". For example,
               if you translate this to "24 sata" or anything else that isn't
"24-hour" or
               "12-hour", things will not work.
              -->
            24-hour

The translation for sl is none of the allowed values:
<default>24-urni</default>