Hello again, Marina,
Apologies, but I gave you incorrect information in my previous post. I did a bit more research into this issue, and it turns out that this is not in fact a software issue but a translation issue. It appears that the current translation for "Sobota" is "sobota". You can see it here if you visit: http://translate.wordpress.com/projects/wpcom/sl/default
Click on "Search" and type in "sobota".
Since translation on WordPress.com is volunteer-based, you can suggest a new translation right at the GlotPress page that I linked (http://translate.wordpress.com/projects/wpcom/sl/default).
Please note that any suggestions you make must be approved before they are live.
If you have any questions about translations, a good place to ask would be our Translations forum: http://en.forums.wordpress.com/forum/translations
Hope this helps! Let me know if you have any other questions.