To Add new locales in the server type

dpkg-reconfigure locales

With the spacebar add locales and Enter to confirm.

    def get_last_revenues_vendor(self):
        vendor_partner = self.env.user.partner_id
        today = datetime.date.today()
        out = []
        try:
            locale.setlocale(locale.LC_ALL, self.env.user.lang) # Set user locale language
        except Exception as ex:
            logging.error('Cannot set locale for %r please install it in the server, error %r' % (self.env.user.lang, ex))
        import calendar
        for month_count in [-3, -2, -1]:
            month_date = today + relativedelta(months=month_count) # Go back of number of months
            days = monthrange(month_date.year, month_date.month) # Get days in given month
            _, number_of_days = days
            start_date = datetime.date(month_date.year, month_date.month, 1)
            end_date = datetime.date(month_date.year, month_date.month, number_of_days)
            purchase_line_ids = self.env['purchase.order.line'].search([
                ('create_date', '>', start_date),
                ('create_date', '<', end_date),
                ])
            amount = 0
            for purchase_line_id in purchase_line_ids:
                if purchase_line_id.order_id.partner_id.id == vendor_partner.id:
                    amount += purchase_line_id.price_subtotal
            # Compute month name using locale and calendar
            out.append('Mese %s %s €' % (calendar.month_name[start_date.month].capitalize(), amount))
        return out