diff --git a/packages/client/src/components/hermes/chat/highlight.ts b/packages/client/src/components/hermes/chat/highlight.ts
index 30a2ae3f..0ffffd5a 100644
--- a/packages/client/src/components/hermes/chat/highlight.ts
+++ b/packages/client/src/components/hermes/chat/highlight.ts
@@ -1,6 +1,62 @@
-import hljs from 'highlight.js'
+import hljs from 'highlight.js/lib/core'
+import bash from 'highlight.js/lib/languages/bash'
+import c from 'highlight.js/lib/languages/c'
+import cpp from 'highlight.js/lib/languages/cpp'
+import csharp from 'highlight.js/lib/languages/csharp'
+import css from 'highlight.js/lib/languages/css'
+import diff from 'highlight.js/lib/languages/diff'
+import go from 'highlight.js/lib/languages/go'
+import xml from 'highlight.js/lib/languages/xml'
+import java from 'highlight.js/lib/languages/java'
+import javascript from 'highlight.js/lib/languages/javascript'
+import json from 'highlight.js/lib/languages/json'
+import kotlin from 'highlight.js/lib/languages/kotlin'
+import lua from 'highlight.js/lib/languages/lua'
+import markdown from 'highlight.js/lib/languages/markdown'
+import php from 'highlight.js/lib/languages/php'
+import python from 'highlight.js/lib/languages/python'
+import r from 'highlight.js/lib/languages/r'
+import ruby from 'highlight.js/lib/languages/ruby'
+import rust from 'highlight.js/lib/languages/rust'
+import scss from 'highlight.js/lib/languages/scss'
+import shell from 'highlight.js/lib/languages/shell'
+import sql from 'highlight.js/lib/languages/sql'
+import swift from 'highlight.js/lib/languages/swift'
+import typescript from 'highlight.js/lib/languages/typescript'
+import yaml from 'highlight.js/lib/languages/yaml'
+import plaintext from 'highlight.js/lib/languages/plaintext'
 import { copyToClipboard } from '@/utils/clipboard'
 
+hljs.registerLanguage('bash', bash)
+hljs.registerLanguage('c', c)
+hljs.registerLanguage('cpp', cpp)
+hljs.registerLanguage('csharp', csharp)
+hljs.registerLanguage('css', css)
+hljs.registerLanguage('diff', diff)
+hljs.registerLanguage('go', go)
+hljs.registerLanguage('xml', xml)
+hljs.registerLanguage('html', xml)
+hljs.registerLanguage('java', java)
+hljs.registerLanguage('javascript', javascript)
+hljs.registerLanguage('js', javascript)
+hljs.registerLanguage('json', json)
+hljs.registerLanguage('kotlin', kotlin)
+hljs.registerLanguage('lua', lua)
+hljs.registerLanguage('markdown', markdown)
+hljs.registerLanguage('php', php)
+hljs.registerLanguage('python', python)
+hljs.registerLanguage('r', r)
+hljs.registerLanguage('ruby', ruby)
+hljs.registerLanguage('rust', rust)
+hljs.registerLanguage('scss', scss)
+hljs.registerLanguage('shell', shell)
+hljs.registerLanguage('sql', sql)
+hljs.registerLanguage('swift', swift)
+hljs.registerLanguage('typescript', typescript)
+hljs.registerLanguage('ts', typescript)
+hljs.registerLanguage('yaml', yaml)
+hljs.registerLanguage('plaintext', plaintext)
+
 const LANGUAGE_ALIASES: Record<string, string> = {
   shellscript: 'bash',
   sh: 'bash',
diff --git a/packages/client/src/components/layout/LanguageSwitch.vue b/packages/client/src/components/layout/LanguageSwitch.vue
index 06cf5493..c856d3b0 100644
--- a/packages/client/src/components/layout/LanguageSwitch.vue
+++ b/packages/client/src/components/layout/LanguageSwitch.vue
@@ -1,6 +1,7 @@
 <script setup lang="ts">
 import { useI18n } from 'vue-i18n'
 import { NSelect } from 'naive-ui'
+import { switchLocale } from '@/i18n'
 
 const { locale } = useI18n()
 
@@ -16,7 +17,8 @@ const options = [
   { label: 'Português', value: 'pt' },
 ]
 
-function handleChange(val: string) {
+async function handleChange(val: string) {
+  await switchLocale(val)
   locale.value = val
   localStorage.setItem('hermes_locale', val)
 }
diff --git a/packages/client/src/i18n/index.ts b/packages/client/src/i18n/index.ts
index ab9c1643..835651b2 100644
--- a/packages/client/src/i18n/index.ts
+++ b/packages/client/src/i18n/index.ts
@@ -1,19 +1,14 @@
 import { createI18n } from 'vue-i18n'
-import { messages } from './messages'
+import { en, loadLocale, supportedLocales } from './messages'
+import type { SupportedLocale } from './messages'
 
 const saved = localStorage.getItem('hermes_locale')
 
-const supportedLocales = ['en', 'zh', 'zh-TW', 'ja', 'ko', 'fr', 'es', 'de', 'pt'] as const
-type SupportedLocale = (typeof supportedLocales)[number]
-
 function resolveLocale(saved: string | null): SupportedLocale {
   if (saved && (supportedLocales as readonly string[]).includes(saved)) {
     return saved as SupportedLocale
   }
 
-  // Normalize a single BCP-47 tag to a supported locale key.
-  // Covers zh-Hant-TW, zh-TW, zh-HK, zh-MO, zh-Hant → zh-TW
-  //        zh-Hans-*, zh-CN, zh-SG, zh            → zh
   function normalize(tag: string): SupportedLocale | null {
     const lower = tag.toLowerCase()
     if (lower.startsWith('zh')) {
@@ -38,9 +33,28 @@ function resolveLocale(saved: string | null): SupportedLocale {
   return 'en'
 }
 
+const locale = resolveLocale(saved)
+
 export const i18n = createI18n({
   legacy: false,
-  locale: resolveLocale(saved),
+  locale: 'en',
   fallbackLocale: 'en',
-  messages,
+  messages: { en },
 })
+
+export async function setupI18n(): Promise<void> {
+  if (locale !== 'en') {
+    const msgs = await loadLocale(locale)
+    if (msgs) {
+      i18n.global.setLocaleMessage(locale, msgs as any)
+    }
+    ;(i18n.global.locale as any).value = locale
+  }
+}
+
+export async function switchLocale(newLocale: string): Promise<void> {
+  const msgs = await loadLocale(newLocale)
+  if (msgs) {
+    i18n.global.setLocaleMessage(newLocale, msgs as any)
+  }
+}
diff --git a/packages/client/src/i18n/messages.ts b/packages/client/src/i18n/messages.ts
index 260a48d1..236d4ca2 100644
--- a/packages/client/src/i18n/messages.ts
+++ b/packages/client/src/i18n/messages.ts
@@ -1,26 +1,9 @@
-import de from './locales/de'
 import en from './locales/en'
-import es from './locales/es'
-import fr from './locales/fr'
-import ja from './locales/ja'
-import ko from './locales/ko'
-import pt from './locales/pt'
-import zh from './locales/zh'
-import zhTW from './locales/zh-TW'
 
-export type LocaleMessages = Record<string, unknown>
+export type LocaleMessages = Record<string, any>
 
-export const rawMessages = {
-  'en': en,
-  'zh': zh,
-  'zh-TW': zhTW,
-  'ja': ja,
-  'ko': ko,
-  'fr': fr,
-  'es': es,
-  'de': de,
-  'pt': pt,
-} satisfies Record<string, LocaleMessages>
+export const supportedLocales = ['en', 'zh', 'zh-TW', 'ja', 'ko', 'fr', 'es', 'de', 'pt'] as const
+export type SupportedLocale = (typeof supportedLocales)[number]
 
 function isPlainObject(value: unknown): value is LocaleMessages {
   return !!value && typeof value === 'object' && !Array.isArray(value)
@@ -42,11 +25,25 @@ export function mergeMessagesWithFallback(
   return merged
 }
 
-export const messages = Object.fromEntries(
-  Object.entries(rawMessages).map(([locale, localeMessages]) => [
-    locale,
-    locale === 'en'
-      ? localeMessages
-      : mergeMessagesWithFallback(en, localeMessages),
-  ]),
-) as typeof rawMessages
+const localeLoaders: Record<string, () => Promise<{ default: LocaleMessages }>> = {
+  'zh': () => import('./locales/zh'),
+  'zh-TW': () => import('./locales/zh-TW'),
+  'ja': () => import('./locales/ja'),
+  'ko': () => import('./locales/ko'),
+  'fr': () => import('./locales/fr'),
+  'es': () => import('./locales/es'),
+  'de': () => import('./locales/de'),
+  'pt': () => import('./locales/pt'),
+}
+
+export { en }
+
+export async function loadLocale(locale: string): Promise<LocaleMessages | null> {
+  if (locale === 'en') return en
+
+  const loader = localeLoaders[locale]
+  if (!loader) return null
+
+  const mod = await loader()
+  return mergeMessagesWithFallback(en, mod.default)
+}
diff --git a/packages/client/src/main.ts b/packages/client/src/main.ts
index 2a10cc14..7641e7b8 100644
--- a/packages/client/src/main.ts
+++ b/packages/client/src/main.ts
@@ -1,7 +1,7 @@
 import { createApp } from 'vue'
 import { createPinia } from 'pinia'
 import router from './router'
-import { i18n } from './i18n'
+import { i18n, setupI18n } from './i18n'
 import App from './App.vue'
 import './styles/global.scss'
 
@@ -36,4 +36,6 @@ const app = createApp(App)
 app.use(createPinia())
 app.use(i18n)
 app.use(router)
-app.mount('#app')
+setupI18n().then(() => {
+  app.mount('#app')
+})
diff --git a/tests/client/highlight-helper.test.ts b/tests/client/highlight-helper.test.ts
index 1d9f7dd0..d241d926 100644
--- a/tests/client/highlight-helper.test.ts
+++ b/tests/client/highlight-helper.test.ts
@@ -5,9 +5,10 @@ const highlightJsMock = vi.hoisted(() => ({
   highlight: vi.fn((content: string, { language }: { language: string }) => ({
     value: `<span class="mock-${language}">${content}</span>`,
   })),
+  registerLanguage: vi.fn(),
 }))
 
-vi.mock('highlight.js', () => ({
+vi.mock('highlight.js/lib/core', () => ({
   default: highlightJsMock,
 }))
 
diff --git a/tests/client/i18n-coverage.test.ts b/tests/client/i18n-coverage.test.ts
index 19ae096f..e50b8426 100644
--- a/tests/client/i18n-coverage.test.ts
+++ b/tests/client/i18n-coverage.test.ts
@@ -1,12 +1,16 @@
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, beforeAll } from 'vitest'
 import { readdirSync, readFileSync } from 'fs'
 import { join, relative } from 'path'
 
 import { changelog } from '@/data/changelog'
-import { messages, rawMessages } from '@/i18n/messages'
+import { loadLocale, supportedLocales } from '@/i18n/messages'
+import en from '@/i18n/locales/en'
+import { createI18n } from 'vue-i18n'
 
 const SOURCE_ROOT = join(process.cwd(), 'packages/client/src')
 
+const allMessages: Record<string, Record<string, unknown>> = { en }
+
 function walkFiles(dir: string, files: string[] = []): string[] {
   for (const entry of readdirSync(dir, { withFileTypes: true })) {
     const path = join(dir, entry.name)
@@ -93,15 +97,23 @@ function labelLength(value: unknown): number {
 }
 
 describe('i18n locale coverage', () => {
-  // Keys that are newly added but not yet translated in all locales
   const ALLOWED_MISSING_KEYS = new Set([
     'changelog.new_0_5_4_7',
     'chat.sessionNotFound',
   ])
 
+  beforeAll(async () => {
+    const results = await Promise.all(
+      supportedLocales.filter(l => l !== 'en').map(async l => {
+        const msgs = await loadLocale(l)
+        if (msgs) allMessages[l] = msgs
+      }),
+    )
+  })
+
   it('defines every statically referenced translation key in the English source locale', () => {
     const missing = collectLiteralTranslationKeys()
-      .filter((key) => !hasPath(rawMessages.en, key))
+      .filter((key) => !hasPath(en, key))
       .filter((key) => !ALLOWED_MISSING_KEYS.has(key))
 
     expect(missing).toEqual([])
@@ -109,7 +121,7 @@ describe('i18n locale coverage', () => {
 
   it('defines every statically referenced translation key in effective runtime messages', () => {
     const requiredKeys = collectLiteralTranslationKeys()
-    const missing = Object.entries(messages).flatMap(([locale, localeMessages]) =>
+    const missing = Object.entries(allMessages).flatMap(([locale, localeMessages]) =>
       requiredKeys
         .filter((key) => !hasPath(localeMessages, key))
         .filter((key) => !ALLOWED_MISSING_KEYS.has(key))
diff --git a/tests/client/sidebar-search.test.ts b/tests/client/sidebar-search.test.ts
index 6a12fe12..f1077fb1 100644
--- a/tests/client/sidebar-search.test.ts
+++ b/tests/client/sidebar-search.test.ts
@@ -42,6 +42,9 @@ vi.mock('vue-i18n', () => ({
   useI18n: () => ({
     t: (key: string) => key,
   }),
+  createI18n: () => ({
+    global: { locale: { value: 'en' }, setLocaleMessage: vi.fn() },
+  }),
 }))
 
 vi.mock('@/composables/useTheme', () => ({