I tested this in Slackware -current (KDE 4.2.1 and qt-r931082 which equals roughly to 4.5-rc1).
Like in that bug report, I created the directory cărţi
and copied a PDF file into it, then started okular from that directory:
eha@darkstar:~/work/cÄrÅ£i$ okular xmltutorial.pdf
okular(4444): Attempt to use QAction "bookmark_action_0" with KXMLGUIFactory!
The warning about KXMLGUIFactory is harmless. Okular starts and displays the PDF.
Note that I do not even have an UTF-8 enabled locale, but I have no problems working with the diacritics in that directory name.