I have seen "CC" tags on a number of cars in different countries.
These are white tags on the grille with the letters "CC" in red.

I think they have something to do with the civil service or diplomatic
corps.

I want to know what this stands for and why people display them.

A very simple answer will do.

Dear spurious,

"CC" stands for "Corps Consulaire" and distinguishes cars of members
of a Consular Corps. These signs are fixed to the car so that the
driver's special status as a diplomat is known to the forces of
control who may not detain him in any way.

Source:

Privacy World Online: Diplomatic Passport and Honorary Consulship
http://www.privacyworld.com/auto/dippp.htm

Best regards,
Scriptor

Thanks.  That was exactly what I was looking for.