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I'm using pl_PL.UTF-8 locale, and gnome-calendar is almost completely in English (apart from strings from GTK+).
The version is gnome-calendar-3.19.92-1.fc24.x86_64
Same issue with French translation
This is fixed in 3.20 git and will be fixed in next gnome-calendar release to be expected on april 13.
Still not fixed in gnome-calendar-3.20.1-1.fc24.x86_64.
Weird, another reboot seems to have fixed the issue. Sorry for the bother.