diff --git a/lib/SimpleSAML/Locale/Language.php b/lib/SimpleSAML/Locale/Language.php
index 3f0387fbdd4756d7e23069e4ee057aaf9d5c8d8e..f843da0565dd48af6c6e29af080c47c594298a47 100644
--- a/lib/SimpleSAML/Locale/Language.php
+++ b/lib/SimpleSAML/Locale/Language.php
@@ -57,9 +57,18 @@ class Language
 
     /**
      * HTTP GET language parameter name.
+     *
+     * @var string
      */
     private $languageParameterName;
 
+    /**
+     * A custom function to use in order to determine the language in use.
+     *
+     * @var callable|null
+     */
+    private $customFunction;
+
 
     /**
      * Constructor