We did this for Indonesia, so the language copy is id_id. We then had problems with our i18n file not working and after talking to Adobe support, they determined it was because JAVA is expecting in_id.
This is where things ended. They just told us to change it to in_id, but this does not seem like the right solution and goes against their own best practice. The other issue, is that our sitemaps are set up with hreflang for Google and we base this setting off of the url structure. Google is also following ISO 639-1 and expects "id". So we have to make code changes if we are going to go with "in"
I am curious what solution others have come up with.