My understanding is that case insensitive filesystems that wish to be portable have a mapping table in the metadata. A quick search showed references to an 'upcase table', although I'm not sure of the accuracy of the source, so I won't link it.

Just because the user changed the system language doesn't mean the system should be expected to change the upcase table though. That operation would need to be very carefully managed; you can't change the rules if there are existing files that would conflict in the new mapping. And you might have symbolic links that matched because of case insensitivity that won't anymore... Pretty tricky.