whats so hard about the word registration? it means that ur car shud have a record in the local system
So if ure driving it without a record in the country its illegal, simple as that.
Legaly speaking
In Pakistan u need to have Pakistani number plates to drive your car legally, so that incase the cars get stolen a complete record is present.
if ure a foriegn national (FN) number plates can be issued. after going thru the process.
Illegaly speaking
Drive it jub tak dill kare. German or any other number plates not specified by Goverment of Pakistan are considered Fancy or Illegal number plates. 
Even embassies are issued with CC (red colord) numbers there not allowed to drive cars with other countries number plates.
If in any case for security reasons they might use illegal plates but for that they have issued NOC (no objection certificates)
People driving with such plates have either orignal plates at home or in the Boot or are driving illegaly/ Unregistered cars.