Could you send us any messages from Bookpedia in your console log. You can view the console log with the program of the same name in your utilities folder, new messages are at the bottom. If you try to the open the multiple add window it should then report the error in the console log. Thank you.
I found the bug in the Italian localization. You can download Bookpedia again and the Italian version of the add multiple window will be fixed. Thank you for the bug report.