Added dutch translations of menu, tabs and index.xml

I have translated several files, in the fresh-site part of the trunk, that generate the menu, the tabs and the index.html file, into dutch. I was not able to test it though because when I created a new site (using forrest seed; forrest run) I was able to see the english, french and german versions. When I changed to ?locale=nl I received an Internal Server Error. So I probably missing something.