In minimal template you can call the translatable document from your main template with the attribute t-lang set to a language code or to a record field.
You also need to re-browse the related records with the proper context if you use fields that are translatable.
For example see below code

<!-- Main template -->
<template id="report_saleorder">
    <t t-call="report.html_container">
        <t t-foreach="docs" t-as="doc">
            <t t-call="sale.report_saleorder_document" t-lang="doc.partner_id.lang">
        </t>
    </t>
</t></template>

<!-- Translatable template -->
<template id="report_saleorder_document">
    <!-- Re-browse of the record with the partner lang -->
    <t t-set="doc" t-value="doc.with_context({'lang':doc.partner_id.lang})">
    <t t-call="report.external_layout">
        <div class="page">
            <div class="oe_structure">
            <div class="row">
                <div class="col-xs-6">
                    <strong t-if="doc.partner_shipping_id == doc.partner_invoice_id">Invoice and shipping address:</strong>
                    <strong t-if="doc.partner_shipping_id != doc.partner_invoice_id">Invoice address:</strong>
                    <div t-field="doc.partner_invoice_id" t-field-options="{&quot;no_marker&quot;: true}">
                &lt;...&gt;
            <div class="oe_structure">
        </div>

</div></div></div></div></div></t></t></template>

Note-The main template calls the translatable template with doc.partner_id.lang as a t-lang parameter.