Hi,
So why is that a problem ?
HikaShop just displays the description without changing it.
It's either Joomla or your HTML editor which changes it.
Try to switch the "editor" setting of the HikaShop configuration to "none" and edit your product. Check that the description is correct, if not set it with the greek letters and then try again.
If it works, the problem is with the HTML editor you're using.
If that doesn't help, try changing the line:
echo JHTML::_('content.prepare',preg_replace('#<hr *id="system-readmore" */>#i','',$this->element->product_description));
to:
echo $this->element->product_description;
in the file "show_default" via the menu Display>Views. That way, it won't go through Joomla to display the description be directly output the data from the database.