Hello @vmarko!
I confirm, I encountered this problem with calling the _load_textdomain_just_in_time function myself. Yes, it occurs when the language selected is not English (in my case, it is Russian). Please fix this problem in the next update, otherwise this message is very crowded debug.log.
There is also another problem with the is_feed function, it also says that the function is called incorrectly: Conditional tags do not work until the request is parsed. Until that moment, they always return false. Additional information can be found on the page «Debugging in WordPress». (This message was added in version 3.1.0.) in /var/www/mysite/data/www/mysite.ru/wp-includes/functions.php on line 6121
