After the K2 update the new calendar does not contain language files. However, time is only in 12-hour format.

Manually attaching a location from cdnjs.cloudflare.com/ajax/libs/flatpickr/2.6.3/l10n/pl.js does not solve the problem ...

Can I quickly improve on my own?

Nevermind :)
The parameter "media/k2/assets/js/k2.backend.js" should be included in the line 710 (in the end of else{...})

time_24hr: true,
locale: 'pl'

If I add the line "administrator/components/com_k2/helpers/html.php" in line 197
$document->addScript('cdnjs.cloudflare.com/ajax/libs/flatpickr/2.6.3/l10n/pl.js');

The idea of a non-native calendar was good but not yet polished.

Hello there and thank you for your feedback.
Can you post this as a feature request in the repo so we can evaluate it?
github.com/getk2/k2

PS: Kudos :)