When Ubuntu 13.04 is installed using the default English locale,
it also created locales for the country/lanugage you are installing from (it takes this information from GEOIP when setting the timezone).
However, the current country (non-English) locale is not a UTF-8 locale, and causes issues with the display of UTF-8 text in the system.
WHO ARE AFFECTED
The Greek localisation is affected. Probably also affected is Cyrillic (Russian, etc).
HOW TO REPRODUCE:
1. Install Ubuntu 13.04 using the default language (English).
You need to get GEOIP to show that you are at least from Greece (GEOIP to Greece), so that the timezone appears to be Athens/Greece. OR, you can select Athens in the timezone settings during installation.
2. Once you are in Ubuntu 13.04, click on the calendar applet.
You get http://imagebin.ubuntu-gr.org/1363895959.png (WRONG, locale strings are probably shown as ISO-8859-7)
It should be like http://i.imgur.com/MHA1b8O.png (GOOD)
WORKAROUND
Install the language support for Greek so that UTF-8 locales are also created.
Or, run
dpkg-reconfigure locales (not verified yet)
LOCALES ON BAD SYSTEM
http://pastebin.ubuntu-gr.org/d2f25103b (Note that the Greek locale is "el_GR", with no reference to .UTF-8/.utf8).
[If above pastebin URL does not work, see attachment below]
REFERENCES
Discussion at Ubuntu-gr (Greece)
http://forum.ubuntu-gr.org/viewtopic.php?f=4&t=26538
Status changed to 'Confirmed' because the bug affects multiple users.