MDL-84983 core: account for invalid locales in intl extension.

Recent PHP versions are now stricter on the `$locale` value passed
to `IntlDateFormatter` class and will throw exceptions for invalid
values (some of which may be present in our language packs).

See: https://github.com/php/php-src/issues/12912

lib/tests/date_test.php

@@ -787,6 +787,12 @@ final class date_test extends advanced_testcase {
                 "%e %b %Y",
                 " 9 окт. 2024",
             ],
+            'Invalid locale' => [
+                1728487000,
+                'xx',
+                '%B %Y',
+                'October 2024',
+            ],
         ];
     }