a JDK bug (http://bugs.sun.com/view_bug.do?bug_id=6369299) causes wrong localized date format in error message of databox, when entering a wrong date for German locale, and possibly others.
can be reproduced:
new java.text.SimpleDateFormat("yyyy-MM-dd", java.util.Locale.GERMANY).toLocalizedPattern() -> uuuu-MM-tt (uuuu does not make any sense)

Sie müssen ein Datum angeben. Format: tt.MM.uuuu
should be -> Sie müssen ein Datum angeben. Format: tt.MM.jjjj

since it is a JDK bug ZK might implement a different an exception or use a different localization library here.

workaround is to override the org.zkoss.zul.Datebox.getLocalizedFormat() on the Datebox component