If you try to pretend that I18N comes down to shoe-horning everything into Unicode, you will turn the language into a joke.
How common will those problems you are describing be by the time this has been implemented? How common are they even now? I haven't yet encountered a unix box where the file names were not in the system locale encoding. On all reasonably up-to-date Linux boxes that I've seen recently, they were in UTF-8 (and the system locale agreed). On both Windows and Mac OS X, filenames are stored in Unicode, so it is always possible to convert them to unicode. So we can't do Unicode-based I18N because there exist a few unix systems with messed-up file systems?
Haskell's Unicode support is a joke because the API designers tried to avoid the issues related to encoding with wishful thinking (i.e. you open a file and you magically get Unicode characters out of it).
OK, that part is purely wishful thinking, but assuming that filenames are text that can be represented in Unicode is wishful thinking that corresponds to 99% of reality. So why can't the remaining 1 percent of reality be fixed instead? Cheers, Wolfgang