In BasePluginInstaller.php

Code:
    protected function loadInstallerLanguageFile($file)
    {
        $lng = $_SESSION['language'];
        $filename = $this->pluginDir . '/installer/langauages/' . $lng . '/' . $file;
        if (file_exists($filename)) {
            require_once($filename);
        }
    }
Should be 'Installer/languages' - so, uppercase I for Installer and a typo on languages.

Also, the indentation on the description in https://docs.zen-cart.com/dev/plugin...anguage_files/ is incorrect but that is a minor issue. Languages should be indented more than Installer