Fix yet another portability problem in new NLS test.

Fix yet another portability problem in new NLS test.

Álvaro reported offlist that his machine was passing the new-in-v19
nls.sql test in "make check" but not in "make installcheck".  On
investigation, the cause turned out to be that he has LANGUAGE set in
his environment, and with (at least recent versions of) glibc that
overrides LC_MESSAGES and friends, as per previous research by Bryan
Green.  "make check" works because pg_regress unsets LANGUAGE before
starting the postmaster, but in installcheck mode we're exposed to
the prevailing value and we lose.

We're already hacking the value of LANGUAGE in this test for Solaris,
so let's just extend that to unsetting LANGUAGE on every other platform.

Reported-by: Álvaro Herrera <alvherre@kurilemu.de>
Diagnosed-by: Andrew Dunstan <andrew@dunslane.net>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/a337896e-5bff-490b-afc9-c545f06c014c@gmail.com
Backpatch-through: 19