Language code of book collection - what does it do?

Since a while ago it’s possible to add a language code and language name to a book collection. I’m wondering what practical implications this has, does anyone know?
Is it tied to interface language, Contents Menu language or device language in any way? I would like to know if there is a way for the user to choose the book collection at the launch of the app and then have the Contents menu language follow this selection.

I was wondering how it would work with 2 different language code systems?
ISO 639-1 - 2 characters for the interface e.g. en
ISO 639-2 - 3 characters for the Book Collection e.g. eng

Should we use ISO 639-2 for the interface? I have had to for Sepedi/Northern Sotho (nso) since ISO 639-1 doesn’t have a code for it.