diff --git a/packages/backend/migration/1695348946091-normalize-locales.js b/packages/backend/migration/1695348946091-normalize-locales.js
new file mode 100644
index 0000000000..3fc656372b
--- /dev/null
+++ b/packages/backend/migration/1695348946091-normalize-locales.js
@@ -0,0 +1,22 @@
+export class NormalizeLocales1695348946091 {
+	name = "NormalizeLocales1695348946091";
+
+	async up(queryRunner) {
+		await queryRunner
+			.query(`SELECT "id", "lang" FROM "note" WHERE "lang" IS NOT NULL`)
+			.then((notes) =>
+				Promise.all(
+					notes.map((note) => {
+						return queryRunner.query(
+							'UPDATE "note" SET "lang" = $1 WHERE "id" = $2',
+							[note.lang.trim().split("-")[0].split("@")[0], note.id],
+						);
+					}),
+				),
+			);
+	}
+
+	async down(queryRunner) {
+		// The original locales are not stored, so migrating back is not possible.
+	}
+}
diff --git a/packages/backend/src/remote/activitypub/models/note.ts b/packages/backend/src/remote/activitypub/models/note.ts
index 5c8807c29d..fa01046f2d 100644
--- a/packages/backend/src/remote/activitypub/models/note.ts
+++ b/packages/backend/src/remote/activitypub/models/note.ts
@@ -314,11 +314,11 @@ export async function createNote(
 		text = note.source.content;
 		if (note.contentMap != null) {
 			const key = Object.keys(note.contentMap)[0];
-			lang = Object.keys(langmap).includes(key) ? key : null;
+			lang = Object.keys(langmap).includes(key) ? key.trim().split("-")[0].split("@")[0] : null;
 		}
 	} else if (note.contentMap != null) {
 		const entry = Object.entries(note.contentMap)[0];
-		lang = Object.keys(langmap).includes(entry[0]) ? entry[0] : null;
+		lang = Object.keys(langmap).includes(entry[0]) ? entry[0].trim().split("-")[0].split("@")[0] : null;
 		text = htmlToMfm(entry[1], note.tag);
 	} else if (typeof note.content === "string") {
 		text = htmlToMfm(note.content, note.tag);
@@ -584,11 +584,11 @@ export async function updateNote(value: string | IObject, resolver?: Resolver) {
 		text = post.source.content;
 		if (post.contentMap != null) {
 			const key = Object.keys(post.contentMap)[0];
-			lang = Object.keys(langmap).includes(key) ? key : null;
+			lang = Object.keys(langmap).includes(key) ? key.trim().split("-")[0].split("@")[0] : null;
 		}
 	} else if (post.contentMap != null) {
 		const entry = Object.entries(post.contentMap)[0];
-		lang = Object.keys(langmap).includes(entry[0]) ? entry[0] : null;
+		lang = Object.keys(langmap).includes(entry[0]) ? entry[0].trim().split("-")[0].split("@")[0] : null;
 		text = htmlToMfm(entry[1], post.tag);
 	} else if (typeof post.content === "string") {
 		text = htmlToMfm(post.content, post.tag);
diff --git a/packages/backend/src/server/api/endpoints/notes/edit.ts b/packages/backend/src/server/api/endpoints/notes/edit.ts
index 9e58021c2f..8d825cf61c 100644
--- a/packages/backend/src/server/api/endpoints/notes/edit.ts
+++ b/packages/backend/src/server/api/endpoints/notes/edit.ts
@@ -379,8 +379,8 @@ export default define(meta, paramDef, async (ps, user) => {
 	}
 
 	if (ps.lang) {
-		ps.lang = ps.lang.trim();
 		if (!Object.keys(langmap).includes(ps.lang.trim())) throw new Error("invalid param");
+		ps.lang = ps.lang.trim().split("-")[0].split("@")[0];
 	} else if (ps.text) {
 		ps.lang = detectLanguage(ps.text);
 	} else {
diff --git a/packages/backend/src/services/note/create.ts b/packages/backend/src/services/note/create.ts
index 80ea6255ea..080925048c 100644
--- a/packages/backend/src/services/note/create.ts
+++ b/packages/backend/src/services/note/create.ts
@@ -280,8 +280,8 @@ export default async (
 		}
 
 		if (data.lang) {
-			data.lang = data.lang.trim();
 			if (!Object.keys(langmap).includes(data.lang.trim())) throw new Error("invalid param");
+			data.lang = data.lang.trim().split("-")[0].split("@")[0];
 		} else if (data.text) {
 			data.lang = detectLanguage(data.text);
 		} else {