I played around with this some.
I used Java Mobi Metadata Editor found here to edit the dictionary language codes:
https://www.mobileread.com/forums/sh...d.php?t=165729
There’s no real issue setting the in-language to LA, it just shows up in Dictionaries settings without an associated language.
The part I was unable to get to work was to open a book with Latin language code (say from Gutenberg) and having it automatically find the Latin dictionary. You have to choose it manually, after which it seems to ‘stick’ to that dictionary. So it lacks the desired slickness.
So what if we try using a ‘supported’ language code?
I tried using Portuguese. The dictionary shows up as expected as Portuguese in Dictionaries settings. And it works fine with ebooks that already have the language code set to Portuguese: it finds and opens my pseudo-Portuguese dictionary automatically.
But I was apparently unable to take the (dual format .mobi) Latin - coded book from Gutenberg and change it to Portuguese using calibre. Whatever calibre is changing, Kindle fails to see it is supposed to be Portuguese and doesn’t automatically choose the pseudo-Portuguese dictionary. So we haven’t really gained anything over just changing the dictionary from en-en to pt-en. Or maybe this has been addressed with a newer version of calibre (the one I had at hand is old because it’s the only thing that runs on my old MacBook Pro).
I suppose the solution is to have calibre do extra work to produce either kf8 or a new dual format mobi somehow (mobi to mobi conversion?).
As far as which language codes are supported, it is apparently pretty broad. I accidentally used ‘lt’ (Latvian) and it showed up as such in Dictionaries settings.
But if Amazon supported Latin (probably 2 lines of source code) it would be a little nicer.