Use RFC3066 instead of the obsoleted RFC1766

Author: tvdijen

src/XML/Assert/LanguageTrait.php

@@ -11,7 +11,7 @@
  */
 trait LanguageTrait
 {
-    private static string $language_regex = '/^([a-z]{2}|[i]-[a-z]+|[x]-[a-z]{1,8})(-[a-z]{1,8})*$/Di';
+    private static string $language_regex = '/^([a-z]{1,8}|[i]-[a-z]+|[x]-[a-z]{1,8})(-[a-z0-9]{1,8})*$/Di';
 
 
     /**

tests/XML/Assert/LanguageTest.php

@@ -44,6 +44,7 @@ public static function provideValidLanguage(): array
             'one part' => [true, 'es'],
             'two parts' => [true, 'en-US'],
             'many parts' => [true, 'es-this-goes-on-forever'],
+            'three-letter primary code' => [true, 'nso'],
             'x-case' => [true, 'x-klingon'],
             'i-case' => [true, 'i-sami-no'],
         ];