diff --git a/README.md b/README.md
index bdd38d5..5477968 100644
--- a/README.md
+++ b/README.md
@@ -109,6 +109,20 @@ await facturapi.invoices.sendByEmail(invoice.id, {
 });
 ```
 
+## API error language
+
+For API V2, request English error messages with `Accept-Language` using the existing client configuration (Node SDK 4.17.0 or later):
+
+```javascript
+import Facturapi from 'facturapi'
+
+const facturapi = new Facturapi('YOUR_API_KEY', {
+  headers: { 'Accept-Language': 'en' },
+})
+```
+
+Use `es` for Spanish. Regional variants such as `en-US` and preferences such as `en;q=0.9, es;q=0.5` are also supported. Missing or unsupported language preferences fall back to Spanish. The setting applies to all requests from this instance; use separate instances for different languages. Codes remain unchanged, and external SAT/PAC messages and uncataloged legacy messages retain their original language. Localization requires server-side API V2 support; it does not translate local SDK errors.
+
 ## Documentation
 
 Visit [docs.facturapi.io](https://docs.facturapi.io).
diff --git a/test/node/runtime-compat.node.test.ts b/test/node/runtime-compat.node.test.ts
index 0e5f97f..b8bd73a 100644
--- a/test/node/runtime-compat.node.test.ts
+++ b/test/node/runtime-compat.node.test.ts
@@ -53,6 +53,7 @@ describe('runtime compatibility (node)', () => {
     const client = new Facturapi('sk_test_123', {
       headers: {
         'x-facturapi-client': 'MCP',
+        'Accept-Language': 'en-US',
         authorization: 'Bearer ignored',
         'content-type': 'text/plain',
       },
@@ -64,6 +65,7 @@ describe('runtime compatibility (node)', () => {
         'Bearer sk_test_123',
       )
       expect(getHeader(options?.headers, 'x-facturapi-client')).toBe('MCP')
+      expect(getHeader(options?.headers, 'Accept-Language')).toBe('en-US')
       expect(getHeader(options?.headers, 'Content-Type')).toBe(
         'application/json',
       )
diff --git a/test/web/runtime-compat.web.test.ts b/test/web/runtime-compat.web.test.ts
index d855cc9..830909d 100644
--- a/test/web/runtime-compat.web.test.ts
+++ b/test/web/runtime-compat.web.test.ts
@@ -66,6 +66,7 @@ describe('runtime compatibility (web simulation)', () => {
     const client = new Facturapi('sk_test_123', {
       headers: {
         'x-facturapi-client': 'MCP',
+        'Accept-Language': 'en-US',
         authorization: 'Bearer ignored',
         'content-type': 'text/plain',
       },
@@ -77,6 +78,7 @@ describe('runtime compatibility (web simulation)', () => {
         'Bearer sk_test_123',
       )
       expect(getHeader(options?.headers, 'x-facturapi-client')).toBe('MCP')
+      expect(getHeader(options?.headers, 'Accept-Language')).toBe('en-US')
       expect(getHeader(options?.headers, 'Content-Type')).toBe(
         'application/json',
       )
@@ -308,6 +310,7 @@ describe('runtime compatibility (web simulation)', () => {
     const client = new Facturapi('sk_test_123', {
       headers: {
         'x-facturapi-client': 'MCP',
+        'Accept-Language': 'en-US',
         'content-type': 'text/plain',
       },
     })
@@ -322,6 +325,7 @@ describe('runtime compatibility (web simulation)', () => {
         'Bearer sk_test_123',
       )
       expect(getHeader(options?.headers, 'x-facturapi-client')).toBe('MCP')
+      expect(getHeader(options?.headers, 'Accept-Language')).toBe('en-US')
 
       return new Response(
         JSON.stringify({