At the very beginning of the song "Snoopy's Christmas" a men's choir
is singing something in german. My question is this:What's the title
and the artist of the song?

I live in Germany.  I am sure that there are very many recordings of 
"O Tannenbaum", no doubt, several for which no royalties are due.
Thus it could be likely that the version used in "Snoopy's Christmas"
was one of these.

Hi there!

The version of "O Tannenbaum" in "A Charlie Brown Christmas" was sung
by the Vince Guaraldi Trio.  You can purchase the movie soundtrack on
www.amazon.com at the following link:

http://www.amazon.com/gp/product/B000000XDJ/qid=1141440190/sr=2-1/ref=pd_bbs_b_2_1/104-7469599-3665525?s=music&v=glance&n=5174

TO Myoarin-ga: Yes, but which version?

Sorry, I don't know.  I am just speculating on the fact that it was
mentioned in the deleted "answer" and clarifications that "O
Tannenbaum" was inserted into or between named artists work.  Since
there were no credits given, that suggests that royalties were not due
to anyone.
Sorry.  I may be wrong, of course.  It's happened before.

Note to Answerers: The "O Tannenbaum" by Vince Guaraldi isn't the
German version the asker was looking. It seems to be more certainly an
older version, probably an iconic recording of the day.