Hi, we have to inherit the corresponding report template to change the title.

You can use the following code to change the title of the invoice report from 'Invoice' to 'FACTURA'.

<template id="custom_report_invoice_document"
        inherit_id="account.report_invoice_document">
    <xpath expr="t[@t-call='web.external_layout']" position="replace">
        <t t-call="web.html_container">
            <div class="mt-5 clearfix">
                <div class="page mb-4">
                    <h2>
                        <h1 t-if="o.move_type == 'out_invoice' and o.state == 'posted'"
                            style="font-size: 40px;  text-align: center; margin-bottom:2px; font-weight: bold;">
                            FACTURA
                        </h1>
                    </h2>
                </div>
            </div>
        </t>
    </xpath>
</template>

Hope it helps