Propagate locale when using app initiated registration URL

Fixes keycloak#13505

Signed-off-by: Thomas Darimont <thomas.darimont@googlemail.com>

services/src/main/java/org/keycloak/protocol/oidc/endpoints/AuthorizationEndpoint.java

@@ -49,6 +49,7 @@
 import org.keycloak.services.messages.Messages;
 import org.keycloak.services.resources.LoginActionsService;
 import org.keycloak.services.util.CacheControlUtil;
+import org.keycloak.services.util.LocaleUtil;
 import org.keycloak.sessions.AuthenticationSessionModel;
 import org.keycloak.util.TokenUtil;
 
@@ -351,6 +352,7 @@ private Response buildRegister() {
 
         AuthenticationProcessor processor = createProcessor(authenticationSession, flowId, LoginActionsService.REGISTRATION_PATH);
         authenticationSession.setClientNote(APP_INITIATED_FLOW, LoginActionsService.REGISTRATION_PATH);
+        LocaleUtil.processLocaleParam(session, realm, authenticationSession);
 
         return processor.authenticate();
     }

testsuite/integration-arquillian/tests/base/src/main/java/org/keycloak/testsuite/util/OAuthClient.java

@@ -1512,6 +1512,10 @@ public String getLoginFormUrl() {
         return this.getLoginFormUrl(this.baseUrl);
     }
 
+    public String getRegisterationsUrl() {
+        return this.getLoginFormUrl(this.baseUrl).replace("openid-connect/auth", "openid-connect/registrations");
+    }
+
     public String getLoginFormUrl(String baseUrl) {
         UriBuilder b = OIDCLoginProtocolService.authUrl(UriBuilder.fromUri(baseUrl));
         if (responseType != null) {

testsuite/integration-arquillian/tests/base/src/test/java/org/keycloak/testsuite/actions/AppInitiatedRegistrationTest.java

@@ -0,0 +1,51 @@
+package org.keycloak.testsuite.actions;
+
+import jakarta.ws.rs.core.UriBuilder;
+import org.jboss.arquillian.graphene.page.Page;
+import org.junit.Before;
+import org.junit.Test;
+import org.keycloak.locale.LocaleSelectorProvider;
+import org.keycloak.representations.idm.RealmRepresentation;
+import org.keycloak.representations.idm.UserRepresentation;
+import org.keycloak.testsuite.AbstractTestRealmKeycloakTest;
+import org.keycloak.testsuite.Assert;
+import org.keycloak.testsuite.admin.ApiUtil;
+import org.keycloak.testsuite.pages.AppPage;
+import org.keycloak.testsuite.pages.RegisterPage;
+
+public class AppInitiatedRegistrationTest extends AbstractTestRealmKeycloakTest {
+
+    @Page
+    protected AppPage appPage;
+
+    @Page
+    protected RegisterPage registerPage;
+
+    @Override
+    public void configureTestRealm(RealmRepresentation testRealm) {
+    }
+
+    @Before
+    public void before() {
+        ApiUtil.removeUserByUsername(testRealm(), "test-user@localhost");
+    }
+
+    @Test
+    public void ensureLocaleParameterIsPropagatedDuringAppInitiatedRegistration() {
+
+        var appInitiatedRegisterUrlBuilder = UriBuilder.fromUri(oauth.getRegisterationsUrl());
+        appInitiatedRegisterUrlBuilder.queryParam(LocaleSelectorProvider.KC_LOCALE_PARAM, "en");
+        var appInitiatedRegisterUrl = appInitiatedRegisterUrlBuilder.build().toString();
+
+        driver.navigate().to(appInitiatedRegisterUrl);
+
+        registerPage.assertCurrent();
+        registerPage.register("first", "last", "test-user@localhost", "test-user", "test","test");
+
+        appPage.assertCurrent();
+
+        UserRepresentation user = testRealm().users().searchByEmail("test-user@localhost", true).get(0);
+        // ensure that the locale was set on the user
+        Assert.assertEquals("en", user.getAttributes().get("locale").get(0));
+    }
+}