diff --git a/lib/rules/template-require-lang-attribute.js b/lib/rules/template-require-lang-attribute.js
index a2c3b31388..fd5ac85185 100644
--- a/lib/rules/template-require-lang-attribute.js
+++ b/lib/rules/template-require-lang-attribute.js
@@ -1,189 +1,4 @@
-// Common valid BCP 47 language tags (not exhaustive, but covers the most common)
-const COMMON_LANG_CODES = new Set([
-  'aa',
-  'ab',
-  'af',
-  'ak',
-  'am',
-  'an',
-  'ar',
-  'as',
-  'av',
-  'ay',
-  'az',
-  'ba',
-  'be',
-  'bg',
-  'bh',
-  'bi',
-  'bm',
-  'bn',
-  'bo',
-  'br',
-  'bs',
-  'ca',
-  'ce',
-  'ch',
-  'co',
-  'cr',
-  'cs',
-  'cu',
-  'cv',
-  'cy',
-  'da',
-  'de',
-  'dv',
-  'dz',
-  'ee',
-  'el',
-  'en',
-  'eo',
-  'es',
-  'et',
-  'eu',
-  'fa',
-  'ff',
-  'fi',
-  'fj',
-  'fo',
-  'fr',
-  'fy',
-  'ga',
-  'gd',
-  'gl',
-  'gn',
-  'gu',
-  'gv',
-  'ha',
-  'he',
-  'hi',
-  'ho',
-  'hr',
-  'ht',
-  'hu',
-  'hy',
-  'hz',
-  'ia',
-  'id',
-  'ie',
-  'ig',
-  'ii',
-  'ik',
-  'io',
-  'is',
-  'it',
-  'iu',
-  'ja',
-  'jv',
-  'ka',
-  'kg',
-  'ki',
-  'kj',
-  'kk',
-  'kl',
-  'km',
-  'kn',
-  'ko',
-  'kr',
-  'ks',
-  'ku',
-  'kv',
-  'kw',
-  'ky',
-  'la',
-  'lb',
-  'lg',
-  'li',
-  'ln',
-  'lo',
-  'lt',
-  'lu',
-  'lv',
-  'mg',
-  'mh',
-  'mi',
-  'mk',
-  'ml',
-  'mn',
-  'mr',
-  'ms',
-  'mt',
-  'my',
-  'na',
-  'nb',
-  'nd',
-  'ne',
-  'ng',
-  'nl',
-  'nn',
-  'no',
-  'nr',
-  'nv',
-  'ny',
-  'oc',
-  'oj',
-  'om',
-  'or',
-  'os',
-  'pa',
-  'pi',
-  'pl',
-  'ps',
-  'pt',
-  'qu',
-  'rm',
-  'rn',
-  'ro',
-  'ru',
-  'rw',
-  'sa',
-  'sc',
-  'sd',
-  'se',
-  'sg',
-  'si',
-  'sk',
-  'sl',
-  'sm',
-  'sn',
-  'so',
-  'sq',
-  'sr',
-  'ss',
-  'st',
-  'su',
-  'sv',
-  'sw',
-  'ta',
-  'te',
-  'tg',
-  'th',
-  'ti',
-  'tk',
-  'tl',
-  'tn',
-  'to',
-  'tr',
-  'ts',
-  'tt',
-  'tw',
-  'ty',
-  'ug',
-  'uk',
-  'ur',
-  'uz',
-  've',
-  'vi',
-  'vo',
-  'wa',
-  'wo',
-  'xh',
-  'yi',
-  'yo',
-  'za',
-  'zh',
-  'zu',
-]);
+const tags = require('language-tags');
 
 const DEFAULT_CONFIG = {
   validateValues: true,
@@ -193,9 +8,7 @@ function isValidLangTag(value) {
   if (!value || !value.trim()) {
     return false;
   }
-  const parts = value.trim().toLowerCase().split('-');
-
-  return COMMON_LANG_CODES.has(parts[0]);
+  return tags(value.trim()).valid();
 }
 
 function parseConfig(config) {
diff --git a/package.json b/package.json
index de20f9800e..719f475e7f 100644
--- a/package.json
+++ b/package.json
@@ -71,6 +71,7 @@
     "eslint-utils": "^3.0.0",
     "estraverse": "^5.3.0",
     "html-tags": "^3.3.1",
+    "language-tags": "^1.0.9",
     "lodash.camelcase": "^4.3.0",
     "lodash.kebabcase": "^4.1.1",
     "requireindex": "^1.2.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index ea58f19332..17861ed207 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -35,6 +35,9 @@ importers:
       html-tags:
         specifier: ^3.3.1
         version: 3.3.1
+      language-tags:
+        specifier: ^1.0.9
+        version: 1.0.9
       lodash.camelcase:
         specifier: ^4.3.0
         version: 4.3.0
@@ -2737,6 +2740,13 @@ packages:
     resolution: {integrity: sha512-9zy9lkjac+TR1c2tG+mkNSVlyOpInnWdSMiue4F+kq8TwJSgv6o8jhLRg8Ho6SnZ9wOYUq/yozts9qQCfk7bIw==}
     engines: {node: '>=18'}
 
+  language-subtag-registry@0.3.23:
+    resolution: {integrity: sha512-0K65Lea881pHotoGEa5gDlMxt3pctLi2RplBb7Ezh4rRdLEOtgi7n4EwK9lamnUCkKBqaeKRVebTq6BAxSkpXQ==}
+
+  language-tags@1.0.9:
+    resolution: {integrity: sha512-MbjN408fEndfiQXbFQ1vnd+1NoLDsnQW41410oQBXiyXDMYH5z505juWa4KUE1LqxRC7DgOgZDbKLxHIwm27hA==}
+    engines: {node: '>=0.10'}
+
   latest-version@9.0.0:
     resolution: {integrity: sha512-7W0vV3rqv5tokqkBAFV1LbR7HPOWzXQDpDgEuib/aJ1jsZZx6x3c2mBI+TJhJzOhkGeaLbCKEHXEXLfirtG2JA==}
     engines: {node: '>=18'}
@@ -6812,6 +6822,12 @@ snapshots:
 
   ky@1.14.3: {}
 
+  language-subtag-registry@0.3.23: {}
+
+  language-tags@1.0.9:
+    dependencies:
+      language-subtag-registry: 0.3.23
+
   latest-version@9.0.0:
     dependencies:
       package-json: 10.0.1
diff --git a/tests/lib/rules/template-require-lang-attribute.js b/tests/lib/rules/template-require-lang-attribute.js
index d7d7685d71..55956986a5 100644
--- a/tests/lib/rules/template-require-lang-attribute.js
+++ b/tests/lib/rules/template-require-lang-attribute.js
@@ -13,6 +13,8 @@ ruleTester.run('template-require-lang-attribute', rule, {
     '<template><html lang="en"></html></template>',
     '<template><html lang="en-US"></html></template>',
     '<template><html lang="DE-BW"></html></template>',
+    '<template><html lang="zh-Hant-HK"></html></template>',
+    '<template><html lang="yue-Hans"></html></template>',
     '<template><html lang={{lang}}></html></template>',
     {
       code: '<template><html lang="de"></html></template>',
@@ -66,6 +68,14 @@ ruleTester.run('template-require-lang-attribute', rule, {
       options: [{ validateValues: true }],
       errors: [{ message: ERROR_MESSAGE }],
     },
+    {
+      // Invalid region subtag: "xx" is not a registered ISO 3166 / BCP 47
+      // region code. Prior to the country-codes port, the rule only
+      // validated the primary subtag and incorrectly accepted this value.
+      code: '<template><html lang="en-XX"></html></template>',
+      output: null,
+      errors: [{ message: ERROR_MESSAGE }],
+    },
     {
       code: '<template><html></html></template>',
       output: null,
@@ -94,6 +104,8 @@ hbsRuleTester.run('template-require-lang-attribute', rule, {
     '<html lang="en"></html>',
     '<html lang="en-US"></html>',
     '<html lang="DE-BW"></html>',
+    '<html lang="zh-Hant-HK"></html>',
+    '<html lang="yue-Hans"></html>',
     '<html lang={{lang}}></html>',
     {
       code: '<html lang="de"></html>',
@@ -146,6 +158,14 @@ hbsRuleTester.run('template-require-lang-attribute', rule, {
       options: [{ validateValues: true }],
       errors: [{ message: ERROR_MESSAGE }],
     },
+    {
+      // Invalid region subtag: "xx" is not a registered ISO 3166 / BCP 47
+      // region code. Prior to the country-codes port, the rule only
+      // validated the primary subtag and incorrectly accepted this value.
+      code: '<html lang="en-XX"></html>',
+      output: null,
+      errors: [{ message: ERROR_MESSAGE }],
+    },
     {
       code: '<html></html>',
       output: null,