Here is a beta of the plugin that should be Python 3 compatible. It isn't one of the plugins I usually use, but, I'd started the basic Python 3 work to remove errors when testing other plugins. I haven't done much testing, but, it seems to get ISBNs from the PDFs and ePubs in my test library.
While doing this, I have also done the work to make the plugin translatable. The attached file "default.po.txt" should be renamed to "default.po" and used for the translations. I recommend using a tool such as
POEdit to do the editing. If anyone does, please post the translated .po file or PM it to me.
If anyone tries this, please report any issues here. And success as well.