Names of months (but not days) always show in English with Windows server

In Calendar block on the Dashboard, I see "December 2018" even in the greek translation. Why?
Days are translated ok. Strings insertdatetime:months_long & template_dlg:months_long are the only ones containing December and they are both already translated to greek ok.
The same happens in MONTHLY VIEW block (it displays November 2018, December 2018, January 2019 in capital letters though) and the Calendar page (Dashboard / ► Site pages / ► Calendar / ► December 2018).
When I add "&strings=1" at the end of the url, it displays no month name and no string name to translate.