The problem is really the jQueryUI datepicker. It seems they added support for en-ZA in version 1.8.9 of jQueryUI. However it also requires a code change in mojoPortal because we have checks for supported languages in the date picker control that need to be updated to include the newly suported languages.
The other problem is that our time picker integration with the datepicker (ie used in the blog) is broken if you use jqueryUI higher than version 1.8.6. So I will have to try and solve that before I can change it to use a newer version.
I was able to reproduce the problem on my local machine and was able to get it working correctly in the Event Calendar using jQueryUI 1.8.9 but, again for now it breaks other things (like the datetime picker in the blog) so I cannot change to use jqueryui 1.8.9 until I resolve the other problems.
That looks like a good change on the script manager, but I would have thought it would be localized by default, other languages seem to work ok with the toolkit datepicker.