If you want to use extensionless urls like www.ihpva.eu/nl, then you have to actually create a physical folder /nl and put a text file there named Default.aspx.
Notice that it already works correctly if you use the url www.ihpva.eu/nl/Default.aspx as you see the different skin and I was able to edit, so if you don't want to create the folders and filesyou can change links from the flag icons to use [nl]/Default.aspx
I would change the login credentials right away. You can always send me credentials if you need me totry it again.
Hope it helps,