Where can I add/edit the product description for each language ? I can't find it in admin section.
When you create a product you will see a long and short description field.
Within each appropriate box you need to write the decription in every language as was mentioned previously
[en] one, two, three [/en]
[de] Eins, Zwei , Drei [/de]
[fr] un, deux, trois [/fr]
What about the shipping calculation. Is it possible to
see the shipping costs in the cart ?
Yes once you set up shipping in shipping methods the available shipping for the products purchased shows within the cart.
What changes do I have to do if I want to give the user
the possibility to order via fax or debit / invoice ?
You need to amend the 'personal' system to suit your needs.
You may find:
http://www.viart.com/manuals.php to be of some use to you
Thanks,
Diane