ui_locales parameter is not set with onLoad: 'check-sso' in keycloak-js adapter

[Redfish90]

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Area

adapter/javascript

Describe the bug

When initializing keycloak-js (latest version) with the locale option and onLoad: 'check-sso', the adapter does not include the ui_locales query parameter in the authentication request. This is inconsistent with the behavior for onLoad: 'login-required', where ui_locales is correctly set. According to OIDC 1.0 specification section 3.1.2.1, the ui_locales parameter should be supported in all authentication requests.

Environment:

• Keycloak server: latest
• keycloak-js: latest
• Browser: Chrome 125
• OS: Windows 11

Version

26.3.0

Regression

• The issue is a regression

Expected behavior

The authentication request should include the ui_locales parameter (e.g., ui_locales=fr) whenever the locale option is set, regardless of whether onLoad is 'login-required' or 'check-sso'.

Actual behavior

When using onLoad: 'check-sso', the authentication request sent to Keycloak does not include the ui_locales parameter, even if the locale option is set. This results in the login screen not being localized according to the specified locale.
In contrast, when using onLoad: 'login-required', the ui_locales parameter is correctly included.

How to Reproduce?

1. Install and configure the latest keycloak-js npm package.
2. Initialize Keycloak with:

   const keycloak = new Keycloak({
     url: 'https://<keycloak-server>/auth',
     realm: 'myrealm',
     clientId: 'myclient',
   });
   
   keycloak.init({
     onLoad: 'check-sso',
     locale: 'fr',
   });

3. Check the authentication request sent to the Keycloak server.
4. Observe that the ui_locales=fr parameter is missing from the query string.

Anything else?

This inconsistency prevents applications using check-sso from correctly localizing the login screen. According to the OIDC specification, this parameter should be available for all authentication flows.

[Redfish90]

I want to add an additional note based on further investigation:

In my application, I use the keycloak.login() method manually (e.g., on a button click). When I specify the locale in the login method like this:

keycloak.login({ locale: 'fr' });

the ui_locales=fr parameter is correctly included in the authentication request, even when using onLoad: 'check-sso' during initialization.

So, the issue only occurs when setting locale via keycloak.init({ onLoad: 'check-sso', locale: 'fr' })—the initial authentication request does not include the ui_locales parameter. But using keycloak.login({ locale: 'fr' }) works as expected.

I hope this clarification helps with triaging and resolving the issue!

[jonkoops]

I don't quite understand why this would be needed. The purpose of check-sso is to check the authentication status of the user without presenting any UI, so why would this parameter need to be sent along? The documentation also explicitly states this applies to login-required only.