diff --git a/packages/frontend-embed/src/boot.ts b/packages/frontend-embed/src/boot.ts
index 987f68005d2f..bae53c3e7fd9 100644
--- a/packages/frontend-embed/src/boot.ts
+++ b/packages/frontend-embed/src/boot.ts
@@ -8,7 +8,7 @@ import 'vite/modulepreload-polyfill';
 
 import '@/style.scss';
 import { createApp, defineAsyncComponent } from 'vue';
-import { setIframeId } from '@/scripts/post-message.js';
+import { setIframeId } from '@/to-be-shared/post-message.js';
 import { parseEmbedParams } from '@/embed-page.js';
 
 const params = new URLSearchParams(location.search);
diff --git a/packages/frontend-embed/src/to-be-shared/i18n.ts b/packages/frontend-embed/src/to-be-shared/i18n.ts
new file mode 100644
index 000000000000..c2f44a33cc47
--- /dev/null
+++ b/packages/frontend-embed/src/to-be-shared/i18n.ts
@@ -0,0 +1,294 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import type { ILocale, ParameterizedString } from '../../../../locales/index.js';
+
+type FlattenKeys<T extends ILocale, TPrediction> = keyof {
+	[K in keyof T as T[K] extends ILocale
+		? FlattenKeys<T[K], TPrediction> extends infer C extends string
+			? `${K & string}.${C}`
+			: never
+		: T[K] extends TPrediction
+			? K
+			: never]: T[K];
+};
+
+type ParametersOf<T extends ILocale, TKey extends FlattenKeys<T, ParameterizedString>> = TKey extends `${infer K}.${infer C}`
+	// @ts-expect-error -- C は明らかに FlattenKeys<T[K], ParameterizedString> になるが、型システムはここでは TKey がドット区切りであることのコンテキストを持たないので、型システムに合法にて示すことはできない。
+	? ParametersOf<T[K], C>
+	: TKey extends keyof T
+		? T[TKey] extends ParameterizedString<infer P>
+			? P
+			: never
+		: never;
+
+type Tsx<T extends ILocale> = {
+	readonly [K in keyof T as T[K] extends string ? never : K]: T[K] extends ParameterizedString<infer P>
+		? (arg: { readonly [_ in P]: string | number }) => string
+		// @ts-expect-error -- 証明省略
+		: Tsx<T[K]>;
+};
+
+export class I18n<T extends ILocale> {
+	private tsxCache?: Tsx<T>;
+
+	constructor(public locale: T) {
+		//#region BIND
+		this.t = this.t.bind(this);
+		//#endregion
+	}
+
+	public get ts(): T {
+		if (_DEV_) {
+			class Handler<TTarget extends ILocale> implements ProxyHandler<TTarget> {
+				get(target: TTarget, p: string | symbol): unknown {
+					const value = target[p as keyof TTarget];
+
+					if (typeof value === 'object') {
+						return new Proxy(value, new Handler<TTarget[keyof TTarget] & ILocale>());
+					}
+
+					if (typeof value === 'string') {
+						const parameters = Array.from(value.matchAll(/\{(\w+)\}/g), ([, parameter]) => parameter);
+
+						if (parameters.length) {
+							console.error(`Missing locale parameters: ${parameters.join(', ')} at ${String(p)}`);
+						}
+
+						return value;
+					}
+
+					console.error(`Unexpected locale key: ${String(p)}`);
+
+					return p;
+				}
+			}
+
+			return new Proxy(this.locale, new Handler());
+		}
+
+		return this.locale;
+	}
+
+	public get tsx(): Tsx<T> {
+		if (_DEV_) {
+			if (this.tsxCache) {
+				return this.tsxCache;
+			}
+
+			class Handler<TTarget extends ILocale> implements ProxyHandler<TTarget> {
+				get(target: TTarget, p: string | symbol): unknown {
+					const value = target[p as keyof TTarget];
+
+					if (typeof value === 'object') {
+						return new Proxy(value, new Handler<TTarget[keyof TTarget] & ILocale>());
+					}
+
+					if (typeof value === 'string') {
+						const quasis: string[] = [];
+						const expressions: string[] = [];
+						let cursor = 0;
+
+						while (~cursor) {
+							const start = value.indexOf('{', cursor);
+
+							if (!~start) {
+								quasis.push(value.slice(cursor));
+								break;
+							}
+
+							quasis.push(value.slice(cursor, start));
+
+							const end = value.indexOf('}', start);
+
+							expressions.push(value.slice(start + 1, end));
+
+							cursor = end + 1;
+						}
+
+						if (!expressions.length) {
+							console.error(`Unexpected locale key: ${String(p)}`);
+
+							return () => value;
+						}
+
+						return (arg) => {
+							let str = quasis[0];
+
+							for (let i = 0; i < expressions.length; i++) {
+								if (!Object.hasOwn(arg, expressions[i])) {
+									console.error(`Missing locale parameters: ${expressions[i]} at ${String(p)}`);
+								}
+
+								str += arg[expressions[i]] + quasis[i + 1];
+							}
+
+							return str;
+						};
+					}
+
+					console.error(`Unexpected locale key: ${String(p)}`);
+
+					return p;
+				}
+			}
+
+			return this.tsxCache = new Proxy(this.locale, new Handler()) as unknown as Tsx<T>;
+		}
+
+		// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+		if (this.tsxCache) {
+			return this.tsxCache;
+		}
+
+		function build(target: ILocale): Tsx<T> {
+			const result = {} as Tsx<T>;
+
+			for (const k in target) {
+				if (!Object.hasOwn(target, k)) {
+					continue;
+				}
+
+				const value = target[k as keyof typeof target];
+
+				if (typeof value === 'object') {
+					result[k] = build(value as ILocale);
+				} else if (typeof value === 'string') {
+					const quasis: string[] = [];
+					const expressions: string[] = [];
+					let cursor = 0;
+
+					while (~cursor) {
+						const start = value.indexOf('{', cursor);
+
+						if (!~start) {
+							quasis.push(value.slice(cursor));
+							break;
+						}
+
+						quasis.push(value.slice(cursor, start));
+
+						const end = value.indexOf('}', start);
+
+						expressions.push(value.slice(start + 1, end));
+
+						cursor = end + 1;
+					}
+
+					if (!expressions.length) {
+						continue;
+					}
+
+					result[k] = (arg) => {
+						let str = quasis[0];
+
+						for (let i = 0; i < expressions.length; i++) {
+							str += arg[expressions[i]] + quasis[i + 1];
+						}
+
+						return str;
+					};
+				}
+			}
+			return result;
+		}
+
+		return this.tsxCache = build(this.locale);
+	}
+
+	/**
+	 * @deprecated なるべくこのメソッド使うよりも ts 直接参照の方が vue のキャッシュ効いてパフォーマンスが良いかも
+	 */
+	public t<TKey extends FlattenKeys<T, string>>(key: TKey): string;
+	/**
+	 * @deprecated なるべくこのメソッド使うよりも tsx 直接参照の方が vue のキャッシュ効いてパフォーマンスが良いかも
+	 */
+	public t<TKey extends FlattenKeys<T, ParameterizedString>>(key: TKey, args: { readonly [_ in ParametersOf<T, TKey>]: string | number }): string;
+	public t(key: string, args?: { readonly [_: string]: string | number }) {
+		let str: string | ParameterizedString | ILocale = this.locale;
+
+		for (const k of key.split('.')) {
+			str = str[k];
+
+			if (_DEV_) {
+				if (typeof str === 'undefined') {
+					console.error(`Unexpected locale key: ${key}`);
+					return key;
+				}
+			}
+		}
+
+		if (args) {
+			if (_DEV_) {
+				const missing = Array.from((str as string).matchAll(/\{(\w+)\}/g), ([, parameter]) => parameter).filter(parameter => !Object.hasOwn(args, parameter));
+
+				if (missing.length) {
+					console.error(`Missing locale parameters: ${missing.join(', ')} at ${key}`);
+				}
+			}
+
+			for (const [k, v] of Object.entries(args)) {
+				const search = `{${k}}`;
+
+				if (_DEV_) {
+					if (!(str as string).includes(search)) {
+						console.error(`Unexpected locale parameter: ${k} at ${key}`);
+					}
+				}
+
+				str = (str as string).replace(search, v.toString());
+			}
+		}
+
+		return str;
+	}
+}
+
+if (import.meta.vitest) {
+	const { describe, expect, it } = import.meta.vitest;
+
+	describe('i18n', () => {
+		it('t', () => {
+			const i18n = new I18n({
+				foo: 'foo',
+				bar: {
+					baz: 'baz',
+					qux: 'qux {0}' as unknown as ParameterizedString<'0'>,
+					quux: 'quux {0} {1}' as unknown as ParameterizedString<'0' | '1'>,
+				},
+			});
+
+			expect(i18n.t('foo')).toBe('foo');
+			expect(i18n.t('bar.baz')).toBe('baz');
+			expect(i18n.tsx.bar.qux({ 0: 'hoge' })).toBe('qux hoge');
+			expect(i18n.tsx.bar.quux({ 0: 'hoge', 1: 'fuga' })).toBe('quux hoge fuga');
+		});
+		it('ts', () => {
+			const i18n = new I18n({
+				foo: 'foo',
+				bar: {
+					baz: 'baz',
+					qux: 'qux {0}' as unknown as ParameterizedString<'0'>,
+					quux: 'quux {0} {1}' as unknown as ParameterizedString<'0' | '1'>,
+				},
+			});
+
+			expect(i18n.ts.foo).toBe('foo');
+			expect(i18n.ts.bar.baz).toBe('baz');
+		});
+		it('tsx', () => {
+			const i18n = new I18n({
+				foo: 'foo',
+				bar: {
+					baz: 'baz',
+					qux: 'qux {0}' as unknown as ParameterizedString<'0'>,
+					quux: 'quux {0} {1}' as unknown as ParameterizedString<'0' | '1'>,
+				},
+			});
+
+			expect(i18n.tsx.bar.qux({ 0: 'hoge' })).toBe('qux hoge');
+			expect(i18n.tsx.bar.quux({ 0: 'hoge', 1: 'fuga' })).toBe('quux hoge fuga');
+		});
+	});
+}
diff --git a/packages/frontend-embed/src/to-be-shared/post-message.ts b/packages/frontend-embed/src/to-be-shared/post-message.ts
new file mode 100644
index 000000000000..44f020f7f9c4
--- /dev/null
+++ b/packages/frontend-embed/src/to-be-shared/post-message.ts
@@ -0,0 +1,51 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export const postMessageEventTypes = [
+	'misskey:shareForm:shareCompleted',
+	'misskey:embed:ready',
+	'misskey:embed:changeHeight',
+] as const;
+
+export type PostMessageEventType = typeof postMessageEventTypes[number];
+
+export interface PostMessageEventPayload extends Record<PostMessageEventType, any> {
+	'misskey:shareForm:shareCompleted': undefined;
+	'misskey:embed:ready': undefined;
+	'misskey:embed:changeHeight': {
+		height: number;
+	};
+}
+
+export type MiPostMessageEvent<T extends PostMessageEventType = PostMessageEventType> = {
+	type: T;
+	iframeId?: string;
+	payload?: PostMessageEventPayload[T];
+}
+
+let defaultIframeId: string | null = null;
+
+export function setIframeId(id: string): void {
+	if (defaultIframeId != null) return;
+
+	if (_DEV_) console.log('setIframeId', id);
+	defaultIframeId = id;
+}
+
+/**
+ * 親フレームにイベントを送信
+ */
+export function postMessageToParentWindow<T extends PostMessageEventType = PostMessageEventType>(type: T, payload?: PostMessageEventPayload[T], iframeId: string | null = null): void {
+	let _iframeId = iframeId;
+	if (_iframeId == null) {
+		_iframeId = defaultIframeId;
+	}
+	if (_DEV_) console.log('postMessageToParentWindow', type, _iframeId, payload);
+	window.parent.postMessage({
+		type,
+		iframeId: _iframeId,
+		payload,
+	}, '*');
+}