Installation: Install the XenForo Dutch language if you haven't already.
In your admin control panel, go to: Appearance > Languages > click "Import a Language" > Choose file > Import as "No Parent" > check "Overwrite language" > choose "Nederlands" > click "Import".
In your admin control panel, go to: Appearance > Languages > click "Import a Language" > Choose file > Import as "No Parent" > check "Overwrite language" > choose "Nederlands" > click "Import".