diff --git a/README.md b/README.md
index 067a32a..8b4985f 100644
--- a/README.md
+++ b/README.md
@@ -80,8 +80,8 @@ new FitDaysClient(opts?: ClientOptions)
 | ------------- | -------------------------------- | ---------------------------------------------------------- |
 | `region`      | `'us'`                           | `'us'` \| `'eu'` \| `'cn'` — picks the API host            |
 | `baseUrl`     | derived from `region`            | overrides the host (useful for tests or new hosts)         |
-| `country`     | `'US'`                           | sent as the `country` query param                          |
-| `language`    | `'en'`                           | sent as the `language` query param                         |
+| `country`     | `'CN'` for `cn`, otherwise `'US'` | sent as the `country` query param                          |
+| `language`    | `'zh'` for `cn`, otherwise `'en'` | sent as the `language` query param                         |
 | `deviceModel` | `'AndroidSDKbuiltforarm64-6.0'`  | sent as the `device_model` query param                     |
 | `clientId`    | generated (uppercase MD5 of UUID) | identifies the client "device"                            |
 | `fetchImpl`   | `globalThis.fetch`               | inject for tests or to use `undici` / `node-fetch`         |
diff --git a/src/client/fitdays-client.ts b/src/client/fitdays-client.ts
index dcd6b3e..6c492a5 100644
--- a/src/client/fitdays-client.ts
+++ b/src/client/fitdays-client.ts
@@ -8,6 +8,7 @@ import {
   DEFAULT_DEVICE_MODEL,
   FULL_SYNC_WINDOW_SECONDS,
   REGION_HOSTS,
+  REGION_LOCALES,
   USER_AGENT,
 } from '../constants/api.js'
 import { FitDaysApiError } from '../errors/api-error.js'
@@ -26,9 +27,11 @@ export class FitDaysClient {
   private readonly fetchImpl: typeof fetch
 
   constructor(opts: ClientOptions = {}) {
-    this.baseUrl = (opts.baseUrl ?? REGION_HOSTS[opts.region ?? 'us']).replace(/\/$/, '')
-    this.country = opts.country ?? 'US'
-    this.language = opts.language ?? 'en'
+    const region = opts.region ?? 'us'
+    const locale = REGION_LOCALES[region]
+    this.baseUrl = (opts.baseUrl ?? REGION_HOSTS[region]).replace(/\/$/, '')
+    this.country = opts.country ?? locale.country
+    this.language = opts.language ?? locale.language
     this.deviceModel = opts.deviceModel ?? DEFAULT_DEVICE_MODEL
     this.clientId = opts.clientId ?? newClientId()
     this.fetchImpl = opts.fetchImpl ?? fetch
diff --git a/src/constants/api.ts b/src/constants/api.ts
index 75957ed..9bb3aec 100644
--- a/src/constants/api.ts
+++ b/src/constants/api.ts
@@ -1,4 +1,4 @@
-import type { Region } from '../types/client.js'
+import type { Region, RegionLocale } from '../types/client.js'
 
 export const APP_VER = '1.21.0'
 export const DEFAULT_DEVICE_MODEL = 'AndroidSDKbuiltforarm64-6.0'
@@ -10,5 +10,11 @@ export const REGION_HOSTS: Record<Region, string> = {
   us: 'https://online-us.fitdays.cn',
 }
 
+export const REGION_LOCALES: Record<Region, RegionLocale> = {
+  cn: { country: 'CN', language: 'zh' },
+  eu: { country: 'US', language: 'en' },
+  us: { country: 'US', language: 'en' },
+}
+
 /** Default sync window (~6 years / 2160 days) used by `syncAll()`. */
 export const FULL_SYNC_WINDOW_SECONDS = 186624000
diff --git a/src/fitdays-api.ts b/src/fitdays-api.ts
index baf22a4..fbb5165 100644
--- a/src/fitdays-api.ts
+++ b/src/fitdays-api.ts
@@ -9,7 +9,7 @@ export {
 export { PASSWORD_SALT, SIGN_SECRET } from './constants/sign.js'
 export { FitDaysApiError } from './errors/api-error.js'
 export type { ApiResponse, LoginData } from './types/api.js'
-export type { ClientOptions, Region, Session } from './types/client.js'
+export type { ClientOptions, Region, RegionLocale, Session } from './types/client.js'
 export type { SignParams } from './types/sign.js'
 export type {
   AccountInfo,
diff --git a/src/tests/fitdays-client.test.ts b/src/tests/fitdays-client.test.ts
index a0804b6..bbd028c 100644
--- a/src/tests/fitdays-client.test.ts
+++ b/src/tests/fitdays-client.test.ts
@@ -80,6 +80,19 @@ describe('FitDaysClient — constructor', () => {
     assert.equal(c.session, null)
   })
 
+  it('defaults country and language to the China locale for the cn region', () => {
+    const c = new FitDaysClient({ region: 'cn' })
+    assert.equal(c.country, 'CN')
+    assert.equal(c.language, 'zh')
+    assert.equal(c.baseUrl, 'https://online.fitdays.cn')
+  })
+
+  it('keeps the US locale for the eu region', () => {
+    const c = new FitDaysClient({ region: 'eu' })
+    assert.equal(c.country, 'US')
+    assert.equal(c.language, 'en')
+  })
+
   it('honours overrides', () => {
     const c = new FitDaysClient({
       clientId: 'AAAA',
diff --git a/src/types/client.ts b/src/types/client.ts
index 9f63abe..080a295 100644
--- a/src/types/client.ts
+++ b/src/types/client.ts
@@ -10,6 +10,11 @@ export type ClientOptions = {
 
 export type Region = 'cn' | 'eu' | 'us'
 
+export type RegionLocale = {
+  country: string
+  language: string
+}
+
 export type Session = {
   refreshToken?: string
   token: string