diff --git a/packages/backend/src/core/NoteCreateService.ts b/packages/backend/src/core/NoteCreateService.ts
index 9f9f6547d1..61abc27ba9 100644
--- a/packages/backend/src/core/NoteCreateService.ts
+++ b/packages/backend/src/core/NoteCreateService.ts
@@ -63,7 +63,6 @@ import { trackPromise } from '@/misc/promise-tracker.js';
 import { isUserRelated } from '@/misc/is-user-related.js';
 import { isNotNull } from '@/misc/is-not-null.js';
 import { IdentifiableError } from '@/misc/identifiable-error.js';
-import { langmap } from '@/misc/langmap.js';
 
 type NotificationType = 'reply' | 'renote' | 'quote' | 'mention';
 
@@ -339,13 +338,6 @@ export class NoteCreateService implements OnApplicationShutdown {
 			data.text = null;
 		}
 
-		if (data.lang) {
-			if (!Object.keys(langmap).includes(data.lang.toLowerCase())) throw new Error('invalid param');
-			data.lang = data.lang.toLowerCase();
-		} else {
-			data.lang = null;
-		}
-
 		let tags = data.apHashtags;
 		let emojis = data.apEmojis;
 		let mentionedUsers = data.apMentions;
diff --git a/packages/backend/src/core/NoteEditService.ts b/packages/backend/src/core/NoteEditService.ts
index 3d8f52947f..8a631fd27d 100644
--- a/packages/backend/src/core/NoteEditService.ts
+++ b/packages/backend/src/core/NoteEditService.ts
@@ -52,7 +52,6 @@ import { isReply } from '@/misc/is-reply.js';
 import { trackPromise } from '@/misc/promise-tracker.js';
 import { isUserRelated } from '@/misc/is-user-related.js';
 import { IdentifiableError } from '@/misc/identifiable-error.js';
-import { langmap } from '@/misc/langmap.js';
 
 type NotificationType = 'reply' | 'renote' | 'quote' | 'mention' | 'edited';
 
@@ -360,13 +359,6 @@ export class NoteEditService implements OnApplicationShutdown {
 			data.text = null;
 		}
 
-		if (data.lang) {
-			if (!Object.keys(langmap).includes(data.lang.toLowerCase())) throw new Error('invalid param');
-			data.lang = data.lang.toLowerCase();
-		} else {
-			data.lang = null;
-		}
-
 		let tags = data.apHashtags;
 		let emojis = data.apEmojis;
 		let mentionedUsers = data.apMentions;
diff --git a/packages/backend/src/core/activitypub/models/ApNoteService.ts b/packages/backend/src/core/activitypub/models/ApNoteService.ts
index d2f5f124f2..204846f7dc 100644
--- a/packages/backend/src/core/activitypub/models/ApNoteService.ts
+++ b/packages/backend/src/core/activitypub/models/ApNoteService.ts
@@ -256,7 +256,7 @@ export class ApNoteService {
 
 		let lang: string | null = null;
 		if (note.contentMap != null) {
-			const key = Object.keys(note.contentMap)[0].toLowerCase();
+			const key = Object.keys(note.contentMap)[0];
 			lang = Object.keys(langmap).includes(key) ? key : null;
 		}
 
@@ -474,7 +474,7 @@ export class ApNoteService {
 
 		let lang: string | null = null;
 		if (note.contentMap != null) {
-			const key = Object.keys(note.contentMap)[0].toLowerCase();
+			const key = Object.keys(note.contentMap)[0];
 			lang = Object.keys(langmap).includes(key) ? key : null;
 		}
 
diff --git a/packages/backend/src/misc/langmap.ts b/packages/backend/src/misc/langmap.ts
index 1d2ec28212..6b6e8b5b75 100644
--- a/packages/backend/src/misc/langmap.ts
+++ b/packages/backend/src/misc/langmap.ts
@@ -376,10 +376,10 @@ export const iso639Langs3 = {
 export const langmapNoRegion = Object.assign({}, iso639Langs1, iso639Langs3);
 
 export const iso639Regional = {
-	'zh-hans': {
+	'zh-Hans': {
 		nativeName: '中文（简体）',
 	},
-	'zh-hant': {
+	'zh-Hant': {
 		nativeName: '中文（繁體）',
 	},
 };
diff --git a/packages/backend/src/server/api/endpoints/notes/edit.ts b/packages/backend/src/server/api/endpoints/notes/edit.ts
index e12c088e39..fa2225667c 100644
--- a/packages/backend/src/server/api/endpoints/notes/edit.ts
+++ b/packages/backend/src/server/api/endpoints/notes/edit.ts
@@ -380,13 +380,6 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				}
 			}
 
-			if (ps.lang) {
-				if (!Object.keys(langmap).includes(ps.lang.toLowerCase())) throw new Error('invalid param');
-				ps.lang = ps.lang.toLowerCase();
-			} else {
-				ps.lang = null;
-			}
-
 			let channel: MiChannel | null = null;
 			if (ps.channelId != null) {
 				channel = await this.channelsRepository.findOneBy({ id: ps.channelId, isArchived: false });
diff --git a/packages/frontend/src/scripts/langmap.ts b/packages/frontend/src/scripts/langmap.ts
index 4eb3067e17..eb23fe2d71 100644
--- a/packages/frontend/src/scripts/langmap.ts
+++ b/packages/frontend/src/scripts/langmap.ts
@@ -376,10 +376,10 @@ export const iso639Langs3 = {
 export const langmapNoRegion = Object.assign({}, iso639Langs1, iso639Langs3);
 
 export const iso639Regional = {
-	'zh-hans': {
+	'zh-Hans': {
 		nativeName: '中文（简体）',
 	},
-	'zh-hant': {
+	'zh-Hant': {
 		nativeName: '中文（繁體）',
 	},
 };