[Xfce-i18n] Add danish translation on www.xfce.org

[scootergrisen]

Hi i have done some translation on www.xfce.org for danish language (da) 
and i think its good enough to go on the website even though its only 
31% translated.

So other people can more easily test the translation without them having 
to run on localhost.

So i think add this to lib/locales.php:
	'da' => 'Danish',

And take translation from 
https://www.transifex.com/xfce/xfce-www/language/da/