The problem is you'd have to pass an annoying extra argument (even if just a NULL) to numerous functions which have no alternative without that argument.
Technically it would be better, especially from a multi-threading point of view. The locale stuff was designed in the 1980's, before multi-threading was a mainstream technique.
Say you have a multi-threaded global server which has to localize something in the context of a session, to the locale of the user making the request.
Still, for thread support, you don't necessarily need a cluttering argument. The locale can be made into a thread-specific variable. In Lisp I would almost certainly prefer for the local to be a dynamic variable. (It would be pretty silly to be passing an argument to influence whether he decimal point is a comma, while the radix of integers is being controlled by *print-base*.)
What you want is for the locale stuff to be broken out into a complete separate library: a whole separate set of loc_* functions: loc_strtod, loc_printf, and so on.*
