I live in Belgium, when configurating my windows live ID a few years ago it was set in French, so, no problem to change it in dutch, just select region in the account and is was set in dutch. So far no problem.
Till a months ago, when I bought a windows phone.
Everything on the windows phone was in dutch, except the marketplace on the phone: It was still in French.
After a good search on the internet, I found that the problem was caused by the language in the x-box account who was still in french.
So after changing the language here, the problem was solved and the market place was in dutch.
But another problem occurs : all apps bought on my windows phone when the market place was in French were no longer visible in the 'windows phone' website under my account : buying history
On the phone : error : c101a7d1 occurs when updating the apps.
Bought and free apps were not able to update anymore.
When I changed the language in the x-box account back in French, all apps are back visible and no error occurs on my windows phone during updates.
Hopefully you can fix this problem so I can read the marketplace in dutch on my windows phone without losing all my apps under my account on the windows phone website !
P.S. : my phone is a Nokia Lumia 800 with windows 7.5