If you run DNN 5.01.00 or higher, and you see this error while trying to update your site settings:

Error: Site Settings is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: Cannot insert the value NULL into column 'DefaultLanguage', table ...

The problem is the Default Language dropdown on the Site Settings page, Advanced Settings, Other Settings is null (empty), and DNN won't let you save the Site Settings with a null in this field. You can't choose a default language because the drop-down is empty.

Here's what to do:
1. Go to the Site menu and choose Languages.
2. Click the "Edit Language" link near the top of the page.
3. Click Enabled and Update - or choose a language and a fallback language - and then click the enabled checkbox.
4. Click Save at the bottom of the Languages page.
5. Now go back to the Site Settings page.
6. Go to Advanced Settings, Other Settings and choose a Default Language.
7. Now you can make your other Site Settings changes and you won't see the error.