Do you only have one GDM on the system? If so as a workaround you can have en_UK as your system default where the GDM would pick up the system default language. Then for each user and sip trigger you can manually specify de_DE-
se-10-1-10-1(config-mailbox)# ccn trigger sip phonenumber 398
se-10-1-10-1(config-trigger)# locale ?
xx_YY xx=language YY=country code
se-10-1-10-1(config-trigger)# locale
se-10-1-10-1# username antyeung language ?
en_US English / United States
se-10-1-10-1# username antyeung language
The only disadvantage is that all GDM's would pick up the system default. But if you only have one GDM, this might work.
Thanks,
Anthony