diff --git a/src/i18n/Data/Intl/IntlLocales.php b/src/i18n/Data/Intl/IntlLocales.php
index 1a672df21..8dd7cd5a8 100644
--- a/src/i18n/Data/Intl/IntlLocales.php
+++ b/src/i18n/Data/Intl/IntlLocales.php
@@ -1605,6 +1605,18 @@ class IntlLocales implements Locales, Resettable
         if (!$lang || !$region) {
             return false;
         }
+
+        // Check the configurable whitelist
+        $localeCode = strtolower($lang) . '_' . strtoupper($region);
+        $locales = $this->getLocales();
+
+        if (array_key_exists($localeCode, $locales)
+            || array_key_exists(strtolower($localeCode), $locales)
+        ) {
+            return true;
+        }
+
+        // Fallback
         return strcasecmp($lang, $region)
             && strcasecmp($lang, $locale)
             && strcasecmp($region, $locale);
diff --git a/tests/php/i18n/i18nTest.php b/tests/php/i18n/i18nTest.php
index cc16807a2..f7547828c 100644
--- a/tests/php/i18n/i18nTest.php
+++ b/tests/php/i18n/i18nTest.php
@@ -376,6 +376,9 @@ class i18nTest extends SapphireTest
         $this->assertFalse(i18n::getData()->validate('en'), 'Short lang format is not valid');
         $this->assertFalse(i18n::getData()->validate('xx_XX'), 'Unknown locale in correct format is not valid');
         $this->assertFalse(i18n::getData()->validate(''), 'Empty string is not valid');
+        $this->assertTrue(i18n::getData()->validate('de_DE'), 'Known locale where language is same as region');
+        $this->assertTrue(i18n::getData()->validate('fr-FR'), 'Known locale where language is same as region');
+        $this->assertTrue(i18n::getData()->validate('zh_cmn'), 'Known locale with all lowercase letters');
     }
 
     public function testTranslate()