Hi all,
The language setting works fine when the interface setting in Calibre is set to English, because Calibre uses that interface setting to define the language in the ePub e-Book you create.
But for all of us in other countries who have another interface setting then English it doesn't work.
The best solution (after my opinion

is that Kovid adds a meta-data field "language" to the meta-data settings of every book in the Calibre library.
We can then set the language per e-Book, which is then used for the language setting in the ePub version.
When Apple issues dictionaries in other languages in the iBooks App it all goes fine, because the language is then recognized.
Hope Kovid reads this