As you have guessed, you get the error since you try to update a version of a node that is derived (it doesn't have content in the current language).
If you had changed the English culture to Norwegian, you wouldn't have had this problem, since all nodes in the system would be changed to norwegian. Now there are some nodes in the system with only english content.
There are no quick fixes for this, apart from making sure each node has content in one of the languages used in the site.
- Check which language the template is in, using WAFContext.Session.LCID. Just print the LCID in the page somewhere.
- Empty the shopping cart if it has any content. Then try to add a new product to the cart. This should create a new Order node, in the current language.