Hello Juan,

For Odoo v11:

For that you can set "Vat Label" in Country of your Company, if it is not set in country then it will default display "TIN"

Here, I have attached screenshot.

Odoo v10 & v9 :

You can inherit external_layout_footer as following :

<template id="external_layout_footer" inherit_id="report.external_layout_footer">
     <xpath expr="// li[@t-if='company.vat']" position="replace">
        <li t-if="company.vat">NIT: <span t-field="company.vat"/></li>
    </xpath>
</template>

Thanks,