ratz We ship the language packs with thunderbird and they are updated for each version.
Since v78.0 the language packs of Thunderbird and lightning have been merged so there are no more specific language files for the calendar since they are now included in the main language files. So if something went wrong, I could understand if some people reported issues with the calendar but not with the main application.
You did a fresh update and I am updating mine for 3 and half years and it works for both of us. I also installed it on a VM that I use for many months now but on which it was never installed before and I did work.
With tomorrow's sync there will be a minor update of Thunderbird (78.0.1) but I don't see any fix related to the issue that was reported here in the release notes.