core: set prehydrated locale based on active backend locale

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/core/templates/base/header_js.html

@@ -1,6 +1,9 @@
+{% load i18n %}
+{% get_current_language as LANGUAGE_CODE %}
+
 <script>
     window.authentik = {};
-    window.authentik.locale = "{{ tenant.default_locale }}";
+    window.authentik.locale = "{{ LANGUAGE_CODE }}";
     window.authentik.config = JSON.parse('{{ config_json|escapejs }}');
     window.authentik.tenant = JSON.parse('{{ tenant_json|escapejs }}');
     window.addEventListener("DOMContentLoaded", () => {