When resetting translations, add general translation keys (#12071)

core/Translation/Translator.php

@@ -194,7 +194,7 @@ class Translator
     public function reset()
     {
         $this->currentLanguage = $this->getDefaultLanguage();
-        $this->directories = array();
+        $this->directories = array(PIWIK_INCLUDE_PATH . '/lang');
         $this->translations = array();
     }