David,

If you are running the DNN site yourself or have admin authority on the box you can create your own cultureinfo & culture classes and register them on the server.  Then you can just copy & rename a language pack to your newly created culture (and edit as needed)

Here are some links,

http://msdn.microsoft.com/en-us/library/ms172469.aspx

http://blog.mediawhole.com/2007/08/creating-custom-cultureinfo.html

David,

the solution suggested by Robert requires modification of the server and you are not guaranteed, the custom locale will be matched during browser detection.

if you simply "add new language" (instead of installing a language pack)" with fallback en-US, no resource files will be created, just the new language will be selectable and use fallback resource files.

David,

I know about the usability problem in DNN 4, that's why we consolidated it in Admin menu with Scope control (Portal - Host - System) in DNN 5.1 (Although the rest of the module in DNN 5 urgently requires UI improvements.