Fix template-require-lang-attribute: validate every BCP47 subtag

Port only validated parts[0] against a small set; upstream validates
every hyphen-separated subtag (region/script). Add the missing
country-codes table and traverse all subtags. lang="en-XX" is now
correctly flagged.

Author: johanrd

lib/rules/template-require-lang-attribute.js

@@ -1,4 +1,9 @@
-// Common valid BCP 47 language tags (not exhaustive, but covers the most common)
+const COUNTRY_CODES = require('../utils/country-codes');
+
+// Common valid BCP 47 primary language subtags (not exhaustive, but covers the most common).
+// Used as a fast-path check for the primary subtag; full validation of every
+// hyphen-separated subtag is performed against COUNTRY_CODES (mirroring
+// ember-template-lint's upstream behavior).
 const COMMON_LANG_CODES = new Set([
   'aa',
   'ab',
@@ -193,9 +198,30 @@ function isValidLangTag(value) {
   if (!value || !value.trim()) {
     return false;
   }
-  const parts = value.trim().toLowerCase().split('-');
+  const normalized = value.trim().toLowerCase();
+  const parts = normalized.split('-');
+
+  // Primary subtag must be a known language code. Keep the original
+  // COMMON_LANG_CODES fast-path so bare tags like "zh" remain valid even
+  // when they are not present in the full country-codes table.
+  if (!COMMON_LANG_CODES.has(parts[0])) {
+    // Fallback: the whole value may be a registered grandfathered/irregular
+    // tag (e.g. "zh-hant", "sgn-gb").
+    return COUNTRY_CODES.includes(normalized);
+  }
+
+  // Validate every remaining hyphen-separated subtag (region / script /
+  // variant) against the full country-codes table, mirroring upstream
+  // ember-template-lint. A whole-string match against COUNTRY_CODES is
+  // also accepted to cover multi-part registered tags.
+  if (parts.length === 1) {
+    return true;
+  }
 
-  return COMMON_LANG_CODES.has(parts[0]);
+  return (
+    parts.slice(1).every((p) => COUNTRY_CODES.includes(p)) ||
+    COUNTRY_CODES.includes(normalized)
+  );
 }
 
 function parseConfig(config) {