diff --git a/src/frontend/init.tsx b/src/frontend/init.tsx
index e1e2a77..e9c3337 100644
--- a/src/frontend/init.tsx
+++ b/src/frontend/init.tsx
@@ -15,15 +15,17 @@ import 'dayjs/locale/ja';
 
 dayjs.extend(relativeTime);
 
-if (!localStorage[LOCALSTORAGE_KEY_LANG]) {
-	localStorage[LOCALSTORAGE_KEY_LANG] = getBrowserLanguage();
+let lng = localStorage[LOCALSTORAGE_KEY_LANG];
+
+if (!lng) {
+	lng = localStorage[LOCALSTORAGE_KEY_LANG] = getBrowserLanguage();
 }
 
 i18n
 	.use(initReactI18next)
 	.init({
 		resources,
-		lng: localStorage[LOCALSTORAGE_KEY_LANG],
+		lng,
 		interpolation: {
 			escapeValue: false // Reactは常にXSS対策をしてくれるので、i18next側では対応不要
 		}
diff --git a/src/frontend/langs/index.ts b/src/frontend/langs/index.ts
index ad4246f..93ef029 100644
--- a/src/frontend/langs/index.ts
+++ b/src/frontend/langs/index.ts
@@ -31,5 +31,5 @@ export type LanguageCode = keyof typeof resources;
 
 export const getBrowserLanguage = () => {
 	const lang = navigator.language.replace('-', '_').toLowerCase();
-	return (Object.keys(resources) as LanguageCode[]).map(l => l.toLowerCase()).find(k => k.startsWith(lang)) ?? 'en_US';
+	return (Object.keys(resources) as LanguageCode[]).find(k => k.toLowerCase().startsWith(lang)) ?? 'en_US';
 };