Hello,
After adding the greek language the products are not dispayed at all from the demo. Do you have any idea what I could do about it?

Hi,

This is a problem of Virtuemart. Unfortunately, the existing products are gone if you add an another language. You need to manually edit all products and translate their titles for the new language one-by one.

Look here:
forum.virtuemart.net/index.php?topic=100378.0

People have been discussing about it already in the official virtuemart forum.
But those are already Sample DATAs, so i would suggest you to forget about them and add your own products.

Regards,
Support