Custom field label

Hi, in the wiki it explains how display custom field, but I can’t find anywhere how to display its label?
Let’s say we create custom field for client with label “Tax Id Number”, then we add value to the client and display it using this code:

<?php echo $invoice->client_custom_tax_id_number; ?>

But how to display the label itself “Tax Id Number”?

It is displayed like any other language strings:

<?php echo lang('invoice_date'); ?>

It doesn’t show anything if I do per your example like

<?php echo lang('client_custom_tax_id_number'); ?>

Do I have to create custom translation manually?

Yes, custom translations have to be added to custom_lang.php

But custom_fields already have a label (albeit not translated). It would be helpful to have that simple label available in the invoice template.