I have an e-book which is having a problem on my Kindle PW. I have set the "on call" dictionary to the Oxford English. When I select a word and try to open the dictionary I get a message the I do not have a dictionary in that language.
Going to Calibre I find the metadata language is shown as English. If I edit the book the metadata shows as en.
However, if I use exiftool to examine the metadata.opf I file to language tags, one "en" and on "ar". And, when I use exiftool on the azw3 file on the Kindle, it shows the language tag as "ar".
How do I fix this? exiftool, while it will display the tags from the azw3, will NOT write them.
If you look at the Supported File Types table (https://exiftool.org/#supported), you will see that exiftool cannot write to ebooks such as epub and azw3, only read their metadata.
Calibre (https://calibre-ebook.com/) is probably your best bet for editing ebook metadata. If you're looking for a command line solution, Calibre also includes several command line programs for dealing with ebooks.
Things are now fine. A bit of blather below, giving the facts, but not an explanation.
I am aware for the limitations on EXITtool, as I stated in my last line. The problem is that I haven't found a way to modify the "language" setting inside the .AZW3 file. Further, when I view the tags on the AZW3 file in the Calibre library the Language is en, the value in the .opf file, which is what I can edit, is eng (or on some books en-US), which don't match that in either AZW3 file in Calibre (en), in the .opf file (eng), or on the Kindle (ar).
However: the act of uninstalling the book from the Kindle, and then reinstalling fixed things, for reasons unknown (to me, at least). The Language show for the AZW3 on the Kindle now shows (en).