fix: normalize i18n registry lookup keys

Issue: #62
User-Visible: no

test/i18n.test.mjs

@@ -5,6 +5,8 @@ import { readdirSync, readFileSync } from 'node:fs';
 import {
   FALLBACK_LANGUAGE_CODE,
   LANGUAGE_REGISTRY,
+  buildLanguageLookup,
+  languageEntry,
   languageOptions,
   normalizeLanguageTag,
   resolveLanguageCode,
@@ -38,6 +40,16 @@ test('i18n: registry codes and English fallback are valid', () => {
   }
 });
 
+test('i18n: canonical regional codes use normalized lookup keys', () => {
+  const regional = { code: 'pt-BR', nativeLabel: 'Português (Brasil)', dictionary: {} };
+  const lookup = buildLanguageLookup([regional]);
+  assert.equal(lookup.get('pt-br'), regional);
+  assert.equal(lookup.get(normalizeLanguageTag('PT_br')), regional);
+  for (const entry of LANGUAGE_REGISTRY) {
+    assert.equal(languageEntry(entry.code.toUpperCase()), entry);
+  }
+});
+
 test('i18n: registry matches frontend and backend locale files', () => {
   const registryCodes = LANGUAGE_REGISTRY.map(({ code }) => code).sort();
   const localeCodes = (url) => readdirSync(url)