[12:42:04] <wm-bb>	 [telegram] <amire80> Odd thing: If I look at https://ru.wikipedia.org/wiki/Special:Recentchanges?uselang=qqx , the date is shown as "2021 (february) 28". If I look at the same page in usual Russian, I see it as "28 февраля 2021", where "февраля" is the "genitive" form, which is the message february-gen, not february as shown in qqx. So the output for the users is correct, but the message shown with uselang=qqx is not correct. So does 
[13:26:36] <wm-bb>	 [telegram] <lucaswerkmeister> I’m guessing it’s because the date format in MessagesRu.php includes 'ymd date' => 'Y xg j', where xg is defined by Language::sprintfDate() to mean the genitive month name; qqx probably falls back to the en date formats, where 'ymd date' => 'Y F j', and F is the regular month name, without genitive
[13:27:06] <wm-bb>	 [telegram] <lucaswerkmeister> i.e. the date format depends on the language, and uselang=qqx selects a date format that doesn’t use the genitive message
[13:34:16] <wm-bb>	 [telegram] <Nikki> I see "28 (february) 2021"
[13:34:47] <wm-bb>	 [telegram] <amire80> Sounds about right, thanks! (re @lucaswerkmeister: I’m guessing it’s because the date format in MessagesRu.php includes 'ymd date' => 'Y xg j', where xg is defined by Language::sprintfDate() to mean the genitive month name; qqx probably falls back to the en date formats, where 'ymd date' => 'Y F j', and F is the regular month name, without genitive)