diff --git a/packages/backend/src/server/web/UrlPreviewService.ts b/packages/backend/src/server/web/UrlPreviewService.ts
index f2be28de0..0917d4b2e 100644
--- a/packages/backend/src/server/web/UrlPreviewService.ts
+++ b/packages/backend/src/server/web/UrlPreviewService.ts
@@ -144,7 +144,7 @@ export class UrlPreviewService {
 
 		return summaly(url, {
 			followRedirects: false,
-			lang: lang ?? 'ja-JP',
+			lang: lang ?? 'en-US',
 			agent: agent,
 			userAgent: meta.urlPreviewUserAgent ?? undefined,
 			operationTimeout: meta.urlPreviewTimeout,
@@ -157,7 +157,7 @@ export class UrlPreviewService {
 		const proxy = meta.urlPreviewSummaryProxyUrl!;
 		const queryStr = query({
 			url: url,
-			lang: lang ?? 'ja-JP',
+			lang: lang ?? 'en-US',
 			userAgent: meta.urlPreviewUserAgent ?? undefined,
 			operationTimeout: meta.urlPreviewTimeout,
 			contentLengthLimit: meta.urlPreviewMaximumContentLength,
diff --git a/packages/frontend/.storybook/preload-locale.ts b/packages/frontend/.storybook/preload-locale.ts
index c823ff9be..ace801b9c 100644
--- a/packages/frontend/.storybook/preload-locale.ts
+++ b/packages/frontend/.storybook/preload-locale.ts
@@ -8,6 +8,6 @@ import locales from '../../../locales/index.js';
 
 await writeFile(
 	new URL('locale.ts', import.meta.url),
-	`export default ${JSON.stringify(locales['ja-JP'], undefined, 2)} as const;`,
+	`export default ${JSON.stringify(locales['en-US'], undefined, 2)} as const;`,
 	'utf8',
 )
diff --git a/packages/frontend/src/config.ts b/packages/frontend/src/config.ts
index fec8bfa16..277dfc12a 100644
--- a/packages/frontend/src/config.ts
+++ b/packages/frontend/src/config.ts
@@ -13,7 +13,7 @@ export const hostname = address.hostname;
 export const url = address.origin;
 export const apiUrl = location.origin + '/api';
 export const wsOrigin = location.origin;
-export const lang = miLocalStorage.getItem('lang') ?? 'ja-JP';
+export const lang = miLocalStorage.getItem('lang') ?? 'en-US';
 export const langs = _LANGS_;
 const preParseLocale = miLocalStorage.getItem('locale');
 export let locale = preParseLocale ? JSON.parse(preParseLocale) : null;
diff --git a/packages/frontend/src/scripts/intl-const.ts b/packages/frontend/src/scripts/intl-const.ts
index aaa4f0a86..0f5960ea1 100644
--- a/packages/frontend/src/scripts/intl-const.ts
+++ b/packages/frontend/src/scripts/intl-const.ts
@@ -5,7 +5,7 @@
 
 import { lang } from '@/config.js';
 
-export const versatileLang = (lang ?? 'ja-JP').replace('ja-KS', 'ja-JP');
+export const versatileLang = (lang ?? 'en-US').replace('ja-KS', 'ja-JP');
 
 let _dateTimeFormat: Intl.DateTimeFormat;
 try {
diff --git a/packages/sw/src/scripts/lang.ts b/packages/sw/src/scripts/lang.ts
index 022e31874..6fccedd74 100644
--- a/packages/sw/src/scripts/lang.ts
+++ b/packages/sw/src/scripts/lang.ts
@@ -13,7 +13,7 @@ class SwLang {
 	public cacheName = `mk-cache-${_VERSION_}`;
 
 	public lang: Promise<string> = get('lang').then(async prelang => {
-		if (!prelang) return 'ja-JP';
+		if (!prelang) return 'en-US';
 		return prelang;
 	});