When I faced the same issue, I have changed the site locale into English, added the calculated column formulae, then reverted the site locale back to German.
I've used the below calculated column formulae to get "MMMM-YYYY" format along with the correct sorting order for Months (January, February, March … etc. )
=REPT(" ";13-MONTH(DateColumn))&TEXT(DateColumn;"mmmm")&"-"&YEAR(DateColumn)
But later faced trouble when entries from different years are present. So, I've changed the format into "YYYY-MMMM" using the below formulae.
=YEAR(DateColumn)&"-"&REPT(" ";13-MONTH(DateColumn))&TEXT(DateColumn;"mmmm")
