FEATURE: 3638 Fusion EEL Helper ${Neos.BackendUser.interfaceLanguage}

closes neos#3638
implemented as discussed in neos#3638

yes you are correct - the `implements ProtectedContextAwareInterface` would not be needed in this case as we rely on ObjectAccess::getProperty, but i implemented it so that in the future `Neos.BackendUser.getInterfaceLanguage()` is still disallowed as one should use `Neos.BackendUser.interfaceLanguage`

Neos.Neos/Classes/Fusion/Helper/BackendUserHelper.php

@@ -0,0 +1,43 @@
+<?php
+namespace Neos\Neos\Fusion\Helper;
+
+/*
+ * This file is part of the Neos.Neos package.
+ *
+ * (c) Contributors of the Neos Project - www.neos.io
+ *
+ * This package is Open Source Software. For the full copyright and license
+ * information, please view the LICENSE file which was distributed with this
+ * source code.
+ */
+
+use Neos\Eel\ProtectedContextAwareInterface;
+use Neos\Flow\Annotations as Flow;
+use Neos\Neos\Service\UserService;
+
+/**
+ * BackendUser helper for translations in backend.
+ */
+class BackendUserHelper implements ProtectedContextAwareInterface
+{
+    private const FROM_EEL_DIRECTLY_CALLABLE_METHODS = [];
+
+    #[Flow\Inject(lazy: false)]
+    protected UserService $userService;
+
+    /**
+     * Returns the interface language the user selected. Will fall back to the default language defined in settings.
+     *
+     * @example Neos.BackendUser.interfaceLanguage
+     */
+    public function getInterfaceLanguage(): string
+    {
+        return $this->userService->getInterfaceLanguage();
+    }
+
+    public function allowsCallOfMethod($methodName)
+    {
+        // checks if method is allowed to be called from EEL directly or should use property access notation
+        return in_array($methodName, self::FROM_EEL_DIRECTLY_CALLABLE_METHODS, true);
+    }
+}

Neos.Neos/Configuration/Settings.yaml

@@ -552,6 +552,7 @@ Neos:
       Neos.Array: Neos\Neos\Fusion\Helper\ArrayHelper
       Neos.Rendering: Neos\Neos\Fusion\Helper\RenderingHelper
       Neos.Caching: Neos\Neos\Fusion\Helper\CachingHelper
+      Neos.BackendUser: Neos\Neos\Fusion\Helper\BackendUserHelper
 
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