Hi, problably it's a some sort of bug.
I did install Polish lang pack as a default forum language.
Problem appears during creating Forum post about event. I get error about wrong date.
When I switch to english, everything seems work just as fine.
I suspect that problem apears when I choose date from the calendar, because dates appears now in Polish localization and names
Example:
(https://i.imgur.com/2EVjbQQ.png)
By default, dates are numerical, like this:
(https://i.imgur.com/6H6Faog.png)
Nothing are logged - Log error is empty.
I have created https://github.com/SimpleMachines/SMF2.1/issues/7359 for this.