Hi,

You can remove t-lang attribute from report template to print the report in same language

Example:

< template id="report_saleorder">
    < t t-call="web.html_container">
        < t t-foreach="docs" t-as="doc">
            < t t-call="sale.report_saleorder_document" t-lang="doc.partner_id.lang" rel="ugc">"/>
        < /t>
    < /t>
< /template>

This is sale order report template in V12. Currently it will be printed in partner language, you can remove t-lang attribute to print in the same language irrespective of partner.

Regards