Try a few different locale name spellings in nls.sql.

Try a few different locale name spellings in nls.sql.

While CI testing in advance of commit 8c498479d suggested that all
Unix-ish platforms would accept 'es_ES.UTF-8', the buildfarm has
a different opinion.  Let's dynamically select something that works,
if possible.

Discussion: https://postgr.es/m/E1vUpNU-000kcQ-1D@gemulon.postgresql.org