See example in report_invoice.xml:

<template id="report_invoice">
    <t t-call="report.html_container">
        <t t-foreach="doc_ids" t-as="doc_id">
            <t t-raw="translate_doc(doc_id, doc_model, 'partner_id.lang', 'account.report_invoice_document')"/>
        </t>
    </t>
</template>

translate_doc() - is a helper used when a report should be translated into a specific lang

A generic report use the default rendering context. If you want a new rendering context containing anything you want to process your data Odoo AbstractModel, a custom module is needed, like this:

from openerp import api, models

class ParticularReport(models.AbstractModel):
    _name = 'report.<<module.reportname>>'
    @api.multi
    def render_html(self, data=None):
        report_obj = self.env['report']
        report = report_obj._get_report_from_name('<<module.reportname>>')
        docargs = {
            'doc_ids': self._ids,
            'doc_model': report.model,
            'docs': self,
        }
        
        ctx = dict(self._context)
        ctx['translatable'] = True
        report_obj = report_obj.with_context(ctx)
        
        return report_obj.render('<<module.reportname>>', docargs)