At least for programs written in C, most (all?) modern Unix-like platforms should include the functionality in the base install. On the language side, C89 requires support for wide and multibyte characters in a conforming libc implementation. And POSIX furthermore requires a locales/iconv system to specify and convert between encodings. Neither of those strictly require that UTF-8 be one of the supported encodings (C89 predates Unicode), but any reasonably modern implementation will include Unicode locales. And if it doesn't, I think at this point you can just consider that to be the system's problem: the current assumption for POSIXy programs is that they will use the system locales, not try to implement their own encoding machinery.
