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/classes/date.php

@@ -819,7 +819,17 @@ class core_date {
           $calendar = IntlGregorianCalendar::createInstance($intltz);
           $calendar->setGregorianChange(PHP_INT_MIN);
 
-          return (new IntlDateFormatter($locale, $date_type, $time_type, $intltz, $calendar, $pattern))->format($timestamp);
+          // We need to account for invalid locales here in recent PHP versions, and ensure we catch errors and switch
+          // back to a default value where an invalid locale is provided.
+          try {
+              $formatter = new IntlDateFormatter($locale, $date_type, $time_type, $intltz, $calendar, $pattern);
+              $result = $formatter->format($timestamp);
+          } catch (Error $e) {
+              $formatter = new IntlDateFormatter(null, $date_type, $time_type, $intltz, $calendar, $pattern);
+              $result = $formatter->format($timestamp);
+          }
+
+          return $result;
         };
 
         // Same order as https://www.php.net/manual/en/function.strftime.php

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',
+            ],
         ];
     }