Originally Posted by kovidgoyal
There's actually nothing wrong with the conversion system as you can verify by converting the book using the command line ebook-convert.
However, I'm guessing you used the GUI to convert. Since the GUI does not store language information, it is passing UND to the conversion system. I'[ve changed that to have it pass instead the language you have set the calibre interface to, which should be a reasonable interim solution.
Can we have a setting in ebook-convert to force the language identifier to be forced to be English in case the original eBook is wrong or undefined?