When using HTML customizations in the self checkout and self checkin interfaces, the content should respect both the language and the library. If the user is not logged in, content should be displayed according to $ENV{OPAC_BRANCH_DEFAULT}. If the user is logged in, content should be displayed according to their home library.
I think the way it currently works makes sense in that it always shows the content for the self_check user's home library, which is the library where the transaction is probably happening. In our consortium each library has a separate self check user, and when they log in to the self check station at their library it always display's their HTML content, no matter if a patron from another library logs in to use sco. I tested changing the call to AdditionalContents() in sco-main.tt to use Branches.GetLoggedInBranchcode as done in bug 36083 for the staff interface pages. With that it shows the self checkout user's branch's content on the patron login screen, and then when a patron with a different home library logs in to self check it changes to show the patron's home library's content. I think always showing the content based on the self checkout user's home library makes sense for our use case. But maybe with a different setup, if all libraries shared a single self check user, then it would make more sense to show the content based on the patron's home library. So maybe it should be configurable somehow.