nl regional tag

First of all thanks for the great plugin. It’s working fine so far.
However I am missing the “nl” language tag for Dutch. There is nl-nl, nl-be and nl-nl_formal but no nl.
I managed to add it by changing and updating the custom field but as soon as I update the post it returns to Dutch formal (which I changed for this purpose).
It would be great if you could add the “nl” tag.
Thanks in advance.
Best PP