weird latex-mode

[Trilobit]

Hi,
I want to write german LaTeX-documents with xemacs, for that I want to 
change the command " that it inserts "` instead of ``.
Therefore I inserted following lines into my custom.el:

(setq TeX-open-quote "\"`")
(setq TeX-close-quote "\"'")

But xemacs inserts `` anyway.
But if I go into the menu 
Options->Advanced(Customize)->Emacs->Wp->Tex->AUC Tex->Latex
and there change the version of Latex from "2" to "2e" or vice versa (it 
depends on what the variable is set before) this feature works correctly.
But it's a bit disturbing to change it everytime I begin to edit an 
Latex document.
Any suggestions what could be wrong with my xemacs(-configuration)?

Thx for advance.

Ps: Im using the current Debian-Linux testing distribution with xemacs 
21.4 installed and Auc-Tex