Hello Sebastiaan,
Are you using any other extensions? I tried to reproduce the issue but it worked fine in my setup.
(K2 v.2.6.6 J! 3.1.1 , SigPro and AdminTools)
Are your categories also for Dutch or All languages?
When using the filter to show the dutch K2 items, nothing happens (browser reloads but doesn't save and show the query). Is this a bug?
Are there any errors in your error_log?