why did the German soldier wack the other one with the stick....

For breaking and talking to the Allies about their troop strength and situation just for some water.

No idea what he said, though.

..Joe

"Something something verräter," and verräter is German for traitor.

In the remake, von Falken says "Keine vergebung für verrat," or "No forgiveness for treachery" before he shoots the soldier who talked, so I imagine something similar was said here.

Essentially, he suspected that Richter, the guy who talked in exchange for water, had done that when he saw Gunn giving him the mess tin full of water, and so he bonked him on the head and left him to die in the hot sun after mumbling that he was a traitor.

I mean, really, how many times will you look under Jabba's manboobs?