In a device with Russian language enabled there is invalid case for header. We see "褋械薪褌褟斜褉褟", "芯泻褌褟斜褉褟", but we should see "褋械薪褌褟斜褉褜", "芯泻褌褟斜褉褜".

I can't find the reason. But maybe you could try some other similar locale or headerDateFormat?
I'll try this and inform you about solution, thanks.
I've researched this issue. Valid solution is
calendar.appearance.headerDateFormat = @"LLLL YYYY";
Thanks!
Most helpful comment
I've researched this issue. Valid solution is
Thanks!