Just go to the CCO, download the german locale, install the locale via the gui or the cli, then restart the tftp service, also dont forget to set the user locale, network locale settings on the phones that you want to enable the new locale, also you may enable the network locale setting on an specific device pool.
Network locale: If users require that country-specific tones be played (on the phone), verify that the locale is installed before configuring the network locale
User locale:If the users require that information be displayed (on the phone) in any language other than English, verify that the locale installer is installed before configuring user locale
You may need to also set up the user locale on the user itself in case the user is a extension mobility user
Cisco Unified Communications Manager uses this locale for extension mobility and the Cisco Unified CM User Options. For Cisco Extension Mobility log on, the locale that is specified here takes precedence over the device and device profile settings.
Cesar Fiestas
Network Engineer