I have a suggestion:
I few years ago a buddy of mine and I did a WP plugin to add Languages via the ISO 639-3 codes to post as custom fields. We created this plugin : http://wordpress.org/extend/plugins/language-code/ – Language Code Classification
It’s purpose was to give a custom field type in which a language atribute for each post could be put. This is the same atribute that DC uses: http://dublincore.org/documents/dcmi-terms/#ses-ISO639-3
So I think someone could just use your plugin… But two things: You can see in the photo: http://www.facebook.com/media/set/?set=a.780983236548.2244402.37702737&l=bb1233e6f8&type=1
the DC:Lang box over-runs on your plugin… It does this with a fresh install of WordPress 3.2.1 and when your plugin is the only one activated. I am viewing it through Google Chrome 13.0.708.107 on OS X 10.6.5
the other thing to note is that your plugin will accept an invalid entry for a DC:Lang attribute this could be improved by providing the user with an auto suggest feature. We implemented this with our Language Code Classification plugin. You can grap the code and repurpose it in your plugin. There is one function which is depreciated and will need updated. But I am not sure how to go around and fix it yet. (nor is it a high priority, because I can just use your plugin.) The auto suggest feature was a bit tricky to implement as we had to deal with diacritics and with not suggesting anything until at least 2 characters were entered….
Additionally the language codes in the ISO 639-3 table can change from time to time. So the table, the auto suggest feature queries should be able to be updated somehow….
PS, There are over 7000 codes and each has a language name or two…. There is a URI set for these things too: http://www.lexvo.org/linkeddata/details.html I don’t know if the URIs are updated as often as the codes themselves or not…
I also Posted this on GitHub: https://github.com/patlockley/Wordpress-Dublin-Core/issues/2
