The <ctype.h> macros are just fundamentally broken in any locale that
has multibyte characters: you cannot squeeze a multibyte character
into an input that is supposed to be either an "unsigned char" or EOF.
Vendors can choose either to violate the spec (say, by interpreting
the "int" input as a Unicode codepoint) or to produce useless results.
It's worth pointing out that the official prototype for the ctype macros
all say they take "int" as an argument, and POSIX says they take as
an argument a "character". So interpreting that argument as a Unicode
codepoint (assuming you're currently in a Unicode locale) is, from my
reading, within the spec.
But it sounds like to me that everyone is on board with sprinkling in
some isascii() calls there where it makes sense.
--Ken