diff --git a/npm/ng-packs/packages/core/locale/src/utils/register-locale.ts b/npm/ng-packs/packages/core/locale/src/utils/register-locale.ts
index e6accdeb8c3..ea5d34ea2e0 100644
--- a/npm/ng-packs/packages/core/locale/src/utils/register-locale.ts
+++ b/npm/ng-packs/packages/core/locale/src/utils/register-locale.ts
@@ -11,6 +11,35 @@ export interface LocaleErrorHandlerData {
 
 let localeMap = {} as { [key: string]: string };
 
+const localeLoaders: Record<string, () => Promise<any>> = {
+  ar: () => import('@angular/common/locales/ar'),
+  cs: () => import('@angular/common/locales/cs'),
+  en: () => import('@angular/common/locales/en'),
+  'en-GB': () => import('@angular/common/locales/en-GB'),
+  es: () => import('@angular/common/locales/es'),
+  de: () => import('@angular/common/locales/de'),
+  fi: () => import('@angular/common/locales/fi'),
+  fr: () => import('@angular/common/locales/fr'),
+  hi: () => import('@angular/common/locales/hi'),
+  hu: () => import('@angular/common/locales/hu'),
+  is: () => import('@angular/common/locales/is'),
+  it: () => import('@angular/common/locales/it'),
+  ja: () => import('@angular/common/locales/ja'),
+  ko: () => import('@angular/common/locales/ko'),
+  pt: () => import('@angular/common/locales/pt'),
+  ro: () => import('@angular/common/locales/ro'),
+  ru: () => import('@angular/common/locales/ru'),
+  sk: () => import('@angular/common/locales/sk'),
+  sl: () => import('@angular/common/locales/sl'),
+  th: () => import('@angular/common/locales/th'),
+  tr: () => import('@angular/common/locales/tr'),
+  vi: () => import('@angular/common/locales/vi'),
+  'zh-Hans': () => import('@angular/common/locales/zh-Hans'),
+  'zh-Hant': () => import('@angular/common/locales/zh-Hant'),
+};
+
+const localeSupportList = Object.keys(localeLoaders);
+
 export interface RegisterLocaleData {
   cultureNameLocaleFileMap?: Record<string, string>;
   errorHandlerFn?: (data: LocaleErrorHandlerData) => any;
@@ -19,29 +48,11 @@ export interface RegisterLocaleData {
 function loadLocale(locale: string) {
   // hard coded list works with esbuild. Source https://github.com/angular/angular-cli/issues/26904#issuecomment-1903596563
 
-  const list = {
-    ar: () => import('@angular/common/locales/ar'),
-    cs: () => import('@angular/common/locales/cs'),
-    en: () => import('@angular/common/locales/en'),
-    'en-GB': () => import('@angular/common/locales/en-GB'),
-    es: () => import('@angular/common/locales/es'),
-    de: () => import('@angular/common/locales/de'),
-    fi: () => import('@angular/common/locales/fi'),
-    fr: () => import('@angular/common/locales/fr'),
-    hi: () => import('@angular/common/locales/hi'),
-    hu: () => import('@angular/common/locales/hu'),
-    is: () => import('@angular/common/locales/is'),
-    it: () => import('@angular/common/locales/it'),
-    pt: () => import('@angular/common/locales/pt'),
-    tr: () => import('@angular/common/locales/tr'),
-    ru: () => import('@angular/common/locales/ru'),
-    ro: () => import('@angular/common/locales/ro'),
-    sk: () => import('@angular/common/locales/sk'),
-    sl: () => import('@angular/common/locales/sl'),
-    'zh-Hans': () => import('@angular/common/locales/zh-Hans'),
-    'zh-Hant': () => import('@angular/common/locales/zh-Hant'),
-  };
-  return list[locale]();
+  if (localeSupportList.indexOf(locale) === -1) {
+    return Promise.reject(new Error(`Cannot find the ${locale} locale file.`));
+  }
+
+  return localeLoaders[locale]();
 }
 
 export function registerLocaleForEsBuild(
@@ -53,12 +64,6 @@ export function registerLocaleForEsBuild(
   return (locale: string): Promise<any> => {
     localeMap = { ...differentLocales, ...cultureNameLocaleFileMap };
     const l = localeMap[locale] || locale;
-    const localeSupportList =
-      'ar|cs|en|en-GB|es|de|fi|fr|hi|hu|is|it|pt|tr|ru|ro|sk|sl|zh-Hans|zh-Hant'.split('|');
-
-    if (localeSupportList.indexOf(l) == -1) {
-      return;
-    }
     return new Promise((resolve, reject) => {
       return loadLocale(l)
         .then(val => {
@@ -93,7 +98,7 @@ export function registerLocale(
       return import(
         /* webpackMode: "lazy-once" */
         /* webpackChunkName: "locales"*/
-        /* webpackInclude: /[/\\](ar|cs|en|en-GB|es|de|fi|fr|hi|hu|is|it|pt|tr|ru|ro|sk|sl|zh-Hans|zh-Hant)\.(mjs|js)$/ */
+        /* webpackInclude: /[/\\](ar|cs|en|en-GB|es|de|fi|fr|hi|hu|is|it|ja|ko|pt|ro|ru|sk|sl|th|tr|vi|zh-Hans|zh-Hant)\.(mjs|js)$/ */
         /* webpackExclude: /[/\\]global|extra/ */
         /* @vite-ignore */
         `@angular/common${localePath}`
diff --git a/npm/ng-packs/packages/core/src/lib/tests/register-locale.spec.ts b/npm/ng-packs/packages/core/src/lib/tests/register-locale.spec.ts
new file mode 100644
index 00000000000..451f55cb024
--- /dev/null
+++ b/npm/ng-packs/packages/core/src/lib/tests/register-locale.spec.ts
@@ -0,0 +1,29 @@
+import { registerLocaleForEsBuild, storeLocaleData } from '../../../locale/src/utils/register-locale';
+
+describe('registerLocaleForEsBuild', () => {
+  it('should always return a promise for unsupported locales', async () => {
+    const errorHandlerFn = vi.fn(({ resolve }) => resolve({ default: null }));
+
+    const result = registerLocaleForEsBuild({ errorHandlerFn })('xx');
+
+    expect(result).toBeInstanceOf(Promise);
+    await expect(result).resolves.toEqual({ default: null });
+    expect(errorHandlerFn).toHaveBeenCalledWith(
+      expect.objectContaining({ locale: 'xx', error: expect.any(Error) }),
+    );
+  });
+
+  it('should load supported locales', async () => {
+    await expect(registerLocaleForEsBuild()('ko')).resolves.toEqual(
+      expect.objectContaining({ default: expect.anything() }),
+    );
+  });
+
+  it('should resolve stored locale data for unsupported locales', async () => {
+    const locale = 'xx-extra';
+    const data = { hello: 'world' };
+    storeLocaleData(data, locale);
+
+    await expect(registerLocaleForEsBuild()(locale)).resolves.toEqual({ default: data });
+  });
+});