In Germany, should Ground and Neutral be connected at the main service panel?
It depends on the supply to that property and its earthing arrangements. It could potentially be TNC, TNC-S, TN-S or TT

If it has an earth spike like you suggested it is probably a TT system. It is important you understand the distinction and I suggest you look up those earthing arrangements mentioned above they are well documented with diagrams.

In TT systems the earth neutral must not be bonded together at any point in the installation. The Live and Neutral alone make their way back to the transformer while the earthing will make its way outside the building separately and onto an earthing spike. 

NOTE:
The following paragraph I have block quoted and this paragraph is a disclaimer against it as it would appear subjective and only anecdotal in the experience of this region. As another user pointed out they have experienced the opposite in regard to the TT systems being historic. It does not alter the overall answer for the user who asked the question. However, I feel it adds context to the region so have left it as is.


  "It should be pointed out that TT systems are a historic system and
  the trend is to phase them out as much as possible as the impedance
  path to earth can sometimes be so poor that it affects the ability of
  the system to earth the system correctly."


So no the Neutral and Earth should not be bonded to each other and secondly no your sub panel should not have its own spike it should be connected to the main earthing bar within the property that will then in turn connect to the earthing spike. This is important because you don't want multiple and different impedance paths to ground. With a TNS/TNC system the supplier provides you with the earthing arrangement for you and will guarantee a (ZE) value.