diff --git a/packages/backend/src/server/web/boot.js b/packages/backend/src/server/web/boot.js
index 022dfab83c..3e31b5d5fa 100644
--- a/packages/backend/src/server/web/boot.js
+++ b/packages/backend/src/server/web/boot.js
@@ -34,17 +34,9 @@
 
 	//#region Detect language & fetch translations
 	if (!localStorage.hasOwnProperty('locale')) {
-		const supportedLangs = LANGS;
 		let lang = localStorage.getItem('lang');
-		if (lang == null || !supportedLangs.includes(lang)) {
-			if (supportedLangs.includes(navigator.language)) {
-				lang = navigator.language;
-			} else {
-				lang = supportedLangs.find(x => x.split('-')[0] === navigator.language);
-
-				// Fallback
-				if (lang == null) lang = 'en-US';
-			}
+		if (lang == null || lang.toString == null || lang.toString() === 'null') {
+			lang = 'ja-JP';
 		}
 
 		const metaRes = await window.fetch('/api/meta', {
@@ -67,12 +59,6 @@
 			return;
 		}
 
-		// for https://github.com/misskey-dev/misskey/issues/10202
-		if (lang == null || lang.toString == null || lang.toString() === 'null') {
-			console.error('invalid lang value detected!!!', typeof lang, lang);
-			lang = 'en-US';
-		}
-
 		const localRes = await window.fetch(`/assets/locales/${lang}.${v}.json`);
 		if (localRes.status === 200) {
 			localStorage.setItem('lang', lang);
diff --git a/packages/frontend/src/config.ts b/packages/frontend/src/config.ts
index 9ce3f8fa15..898d5b8177 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 = url + '/api';
 export const wsUrl = url.replace('http://', 'ws://').replace('https://', 'wss://') + '/streaming';
-export const lang = miLocalStorage.getItem('lang') ?? 'en-US';
+export const lang = miLocalStorage.getItem('lang') ?? 'ja-JP';
 export const langs = _LANGS_;
 const preParseLocale = miLocalStorage.getItem('locale');
 export let locale = preParseLocale ? JSON.parse(preParseLocale) : null;
diff --git a/packages/sw/src/scripts/lang.ts b/packages/sw/src/scripts/lang.ts
index dbe25130b4..c8ce36531a 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 'en-US';
+		if (!prelang) return 'ja-JP';
 		return prelang;
 	});