I was pretty surprised and happy to see that there's also a German installer available for System Shock 2. My question is, is the German version the censored one (green blood, some missing "shock" effects, ...), or is it the uncut version, only with German language?

Censored, SS2Tool makes it uncut.

Did make the SS2Tool the german version really uncut or just the blood?

Hmm...Ok. Then I'll definitely play the english version. Is there maybe a language patch, to make the english version german?

Yes, although it's technically not entirely legal.

http://www.systemshock.org/index.php/topic,1634.0.html

if you change the blood color dont forget to change the shock.cfg file

Open the shock.cfg file located in your SShock2 directory using any
text editor you like and add the line (of course without the quotes)

"menschveracht"

before the line containing "TerrTextures16".

Furthermore this re-enables some removed details such as the "hangman"
in the the first level (please read the "known limitations" section!).

Now it is done! The censorship is no longer and the german translation is
still untouched.

What I ask myself is BEWARE SPOILERS

does the head on the Bridge still fly through the room in the original german version?

the head flies through the room.

there a other things that had been changed between the german and the english gog version.

here is a link to a comparison.
its in german but the pictures are selfspeaking.

http://www.schnittberichte.com/schnittbericht.php?ID=618318