I found one way of showing it in the language I want to show it it in, Japanese.
On your site navigate to Site settings -> Site Administration -> Regional settings
On the Regional settings page switch your Local to Greek and click ok. Now you calendar view should be in Greek. That is the solution and the problem, at least for me. It now will be in Greek "Only". so even if you switch back to English user interface the Calendar view will not follow the change and still show your day names in the selected Regional Settings...