One thing I don't think belongs in this library is the ~p plural.

That's very specific to English.  And even in English, its sense
is reversed when you're pluralizing verbs instead of nouns (there,
the *singular* most frequently has the 's' added and the *plural*
is most frequently without it).

This stuff should be locale-dependent, or possibly dynamic, or possibly
part of a different library altogether.

To really address it adequately you'd need a semantics that allowed
function calls determined by the contents of the format string, so
somebody could write, eg:

(let ((format-e (lambda (arg) (if (> arg 1) "s" "")))
      (format-b (lambda (arg) (if (> arg 1) "" "s")))
      (format-c (lambda (arg) (if (> arg 1) "ves" "f"))))
   (format "The wol~c chase~b the boy~e across the snow."
             wolfcount wolfcount boycount))

which makes the whole thing into effectively a dynamic-dispatch
system.  Then people working with other languages could define
different format-* forms specific to each target language and take
into account things like masculine vs. feminine, nominative vs.
genitive, and so on.

But I don't think you really want to bite off language-specific
stuff with this library.

				Bear