form_validation and languages

[eluser]Unknown[/eluser]
Is there any way to concat lang vars in form_validation.php?
I want something like this:

Code:

$config = array(
        'register' => array(
                array(
                        'field' => 'fieldname',
                        'label' => 'lang:register_tag1' . 'lang:register_tag2',
                        'rules' => 'required'
                ),
[...]

Concatenation seems not to work properly, maybe something is wrong, the output is wrong: "register_tag1 register_tag2".
Any help? Thanks in advance!