Site Management site not working when you delete only available language
I know this sounds weird and should never be really an issue. But when having a single language TYPO3 instance with a working site configuration and you delete the only language configuration from that it won't let you save the site configuration because "Available languages for this site" is empty and cannot be (re)added.
If you remove that language configuration it should be readded back to the list.
The only workaround for this is to completely delete the site configuration and start over.
Tested only on TYPO3 9.5.4 running nginx with PHP 7.3.