Works on:
rundeck-2.5.1-1-GA
Ubuntu 14.04.2 LTS (trusty)
cd /var/lib/rundeck/exp/webapp/WEB-INF/grails-app/i18n
cp messages.properties messages_de.properties
# find and edit following lines:
cat messages_de.properties | grep jobslist\.date\.format
jobslist.date.format=DD/MM/YYYY HH:mm
jobslist.date.format.ko=DD/MM/YYYY HH:mm