In \phpbb\template\twig\extension.php, function lang() performs full template data and then a part ofit ('rootref') copying in https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/phpbb/template/twig/extension.php#L172-L173

		$context = $this->context->get_data_ref();

		$context_vars = $context['.'][0];

In respect of optimization, this can be shortened to

$context_vars = $this->context->get_root_ref();

(the latter also returns a referense).