In our Moodle installation workshops we found out that a Moodle installation with de_du_utf8 produces english role names. This is wrong. Both the installation with de_utf8 and with de_du_utf8 should make german role names.
There will be another mistake when you choose de_du_utf8 with your installation .... the installer asks you during the process to load the language package de_du_utft8 .... but if you load only the incomplete de_du_utf8 you also need the parent package de_utf8 ... and this will still be missing after your installation with de_du_utf8.
I think it would be a very easy and quick bug repair to leave away de_du_utf8 from the installer.