tests: add tests for locale ordering

* tests/sort/sort-locale.sh: Check sort,ls have the same (non C) order.
* tests/local.mk: Reference the new test.

tests/local.mk

@@ -413,6 +413,7 @@ all_tests =					\
   tests/sort/sort-files0-from.pl		\
   tests/sort/sort-float.sh			\
   tests/sort/sort-h-thousands-sep.sh		\
+  tests/sort/sort-locale.sh			\
   tests/sort/sort-merge.pl			\
   tests/sort/sort-merge-fdlimit.sh		\
   tests/sort/sort-month.sh			\

tests/sort/sort-locale.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+# Test sort locale ordering
+
+# Copyright (C) 2025 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ sort ls
+
+check_different_from_C() {
+  test "$(printf '%s\n' "$1" "$2" | LC_ALL=C sort)" != \
+       "$(printf '%s\n' "$1" "$2" | sort)"
+}
+
+check_hard_collate() {
+  # Correlate with ls
+  touch "$1" "$2" || framework_failure_
+  test "$(printf '%s\n' "$1" "$2" | sort)" = "$(ls -1 "$1" "$2")" || fail=1
+
+  if test "$fail" != 1; then
+    check_different_from_C "$1" "$2" ||
+      skip_ "Strings '$1' '$2' sort the same in C and $LC_ALL locales"
+  fi
+}
+
+export LC_ALL=en_US.iso8859-1  # only lowercase form works on macOS 10.15.7
+if test "$(locale charmap 2>/dev/null | sed 's/iso/ISO-/')" = ISO-8859-1; then
+  check_hard_collate 'a_a' 'a b'  # underscore and space considered equal
+  check_hard_collate 'aaa' 'BBB'  # case insensitive ordering
+  check_hard_collate "$(printf 'aa\xe9')" 'aaf'  # é comes before f
+fi
+
+export LC_ALL=$LOCALE_FR_UTF8
+if test "$(locale charmap 2>/dev/null)" = UTF-8; then
+  check_hard_collate 'aaé' 'aaf'  # é comes before f
+  check_hard_collate 'aéY' "$(printf 'ae\u0301Z')"  # NFC/NFD é are equal
+fi
+
+Exit $fail