> Instead, open() determines the default encoding by calling the same function that's used to initialize Py_FileSystemDefaultEncoding: get_locale_encoding() in Python/pythonrun.c.  Which on POSIX systems calls the POSIX function nl_langinfo().

open() will use at default the encoding of nl_langinfo() as sys.getfilesystemencoding() does on *nix. This is the part that looks dirty to me. As soon as LANG is set to C open() will rely on 'ascii' due to nl_langinfo() like sys.getfilesystemencoding() does too.