docs: show how to select API error language

Author: javorosas

README.es.md

@@ -116,13 +116,42 @@ Facturapi facturapi = Facturapi.builder("sk_test_...")
 Para pruebas o configuraciones avanzadas, puedes inyectar tu propio `OkHttpClient`:
 
 ```java
+import io.facturapi.http.FacturapiConfig;
 import okhttp3.OkHttpClient;
 
-Facturapi facturapi = Facturapi.builder("sk_test_...")
-  .httpClient(new OkHttpClient())
-  .build();
+Facturapi facturapi = new Facturapi(
+  FacturapiConfig.builder("sk_test_...")
+    .httpClient(new OkHttpClient())
+    .build());
 ```
 
+## Idioma de los errores de la API
+
+En API V2 puedes solicitar mensajes de error en inglés con `Accept-Language`. Usa la configuración existente del cliente:
+
+```java
+import io.facturapi.Facturapi;
+import io.facturapi.http.FacturapiConfig;
+import java.time.Duration;
+import okhttp3.OkHttpClient;
+
+OkHttpClient httpClient = new OkHttpClient.Builder()
+    .connectTimeout(Duration.ofSeconds(30))
+    .callTimeout(Duration.ofSeconds(30))
+    .addInterceptor(chain -> chain.proceed(
+        chain.request().newBuilder()
+            .header("Accept-Language", "en")
+            .build()))
+    .build();
+
+Facturapi facturapi = new Facturapi(
+    FacturapiConfig.builder("YOUR_API_KEY")
+        .httpClient(httpClient)
+        .build());
+```
+
+Usa `es` para español. También se aceptan variantes como `en-US` y preferencias como `en;q=0.9, es;q=0.5`. Sin el header o sin un idioma compatible, la API usa español. La preferencia se aplica a todas las solicitudes de esta instancia; usa instancias separadas si necesitas varios idiomas. Los códigos no cambian y los mensajes externos SAT/PAC y los mensajes legacy no catalogados conservan su idioma original. La localización depende del soporte de API V2 en el servidor; no traduce errores locales del SDK.
+
 ## Documentación
 
 Docs completas: [https://docs.facturapi.io](https://docs.facturapi.io)

README.md

@@ -116,13 +116,42 @@ Facturapi facturapi = Facturapi.builder("sk_test_...")
 For tests or advanced setups, you can inject your own `OkHttpClient`:
 
 ```java
+import io.facturapi.http.FacturapiConfig;
 import okhttp3.OkHttpClient;
 
-Facturapi facturapi = Facturapi.builder("sk_test_...")
-  .httpClient(new OkHttpClient())
-  .build();
+Facturapi facturapi = new Facturapi(
+  FacturapiConfig.builder("sk_test_...")
+    .httpClient(new OkHttpClient())
+    .build());
 ```
 
+## API error language
+
+For API V2, request English error messages with `Accept-Language` using the existing client configuration:
+
+```java
+import io.facturapi.Facturapi;
+import io.facturapi.http.FacturapiConfig;
+import java.time.Duration;
+import okhttp3.OkHttpClient;
+
+OkHttpClient httpClient = new OkHttpClient.Builder()
+    .connectTimeout(Duration.ofSeconds(30))
+    .callTimeout(Duration.ofSeconds(30))
+    .addInterceptor(chain -> chain.proceed(
+        chain.request().newBuilder()
+            .header("Accept-Language", "en")
+            .build()))
+    .build();
+
+Facturapi facturapi = new Facturapi(
+    FacturapiConfig.builder("YOUR_API_KEY")
+        .httpClient(httpClient)
+        .build());
+```
+
+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
 
 Full docs: [https://docs.facturapi.io](https://docs.facturapi.io)

src/test/java/io/facturapi/FacturapiHttpClientTest.java

@@ -17,9 +17,13 @@ void sendsBearerAuthAndParsesTypedJson() throws Exception {
     StubHttpClient httpClient = new StubHttpClient();
     httpClient.enqueueJson(200, "{\"ok\":true}");
 
+    var builder = httpClient.client().newBuilder();
+    builder.interceptors().add(0, chain -> chain.proceed(
+      chain.request().newBuilder().header("Accept-Language", "en-US").build()
+    ));
     FacturapiHttpClient client = new FacturapiHttpClient(
       FacturapiConfig.builder("sk_test_123")
-        .httpClient(httpClient.client())
+        .httpClient(builder.build())
         .build()
     );
 
@@ -28,6 +32,7 @@ void sendsBearerAuthAndParsesTypedJson() throws Exception {
 
     var request = httpClient.requests().get(0);
     assertEquals("Bearer sk_test_123", request.headers().firstValue("Authorization").orElse(""));
+    assertEquals("en-US", request.headers().firstValue("Accept-Language").orElse(""));
     assertEquals("POST", request.method());
     assertEquals("/v2/invoices?page=1", request.uri().getPath() + "?" + request.uri().getQuery());
   }