Thanks for that patch: it's a good move forward for i18n. Some suggestions:

* Today I fixed the bug with "%%" and the 'error' function, so there's no need 
for a FIXME or a workaround any more.

* In strings.texi, reorder the format spec description so that it matches the 
textual order of a format spec. This should lessen confusion.

* Allow field numbers in a %% spec. All other components of a format spec are 
allowed in %%, so odd to report an error for just field numbers.

* There is no need for a special diagnostic for field numbers greater than 
PTRDIFF_MAX. Just use the same diagnostic other too-large field numbers use. 
This avoids a need for an alloca.

* Reword "Invalid field number `0'" to "Invalid format field number 0" to make 
it more obvious that it's a format and there's no need to quote the 0.

Proposed further patch attached (it addresses the above points), along with a 
copy of your patch rebased to current master for convenience.