I'm trying to set the culture of a website to en-ZM in the web.config of the site but I get an error:

The <globalization> tag contains an invalid value for the 'culture' attribute.

When I try to set it via IIS' interface, it seems Zambia isn't listed in the available options, although other African countries like Zimbabwe are (en-ZW).

Example of the .NET Globalization UI in IIS 8.5 on server 1 (Windows Server 2012R2):

Example of the .NET Globalization UI in IIS 10 on server 2 (Windows Server 2016):

My local machine is running Win10 and has this culture available for configuration so I hadn't expected this to be an issue. Is there any update I can manually install to add the missing cultures?