Add a regression test to verify that NLS translation works.

Add a regression test to verify that NLS translation works.

We've never actually had a formal test for this facility.
It seems worth adding one now, mainly because we are starting
to depend on gettext() being able to handle the PRI* macros
from <inttypes.h>, and it's not all that certain that that
works everywhere.  So the test goes to a bit of effort to
check all the PRI* macros we are likely to use.

(This effort has indeed found one problem already, now fixed
in commit f8715ec86.)

Discussion: https://postgr.es/m/3098752.1765221796@sss.pgh.pa.us
Discussion: https://postgr.es/m/292844.1765315339@sss.pgh.pa.us