Hi,

You can add the Salesperson in the invoice report in two ways

1)a. First you need to locate the actual invoice layout template of the specific country localization from the addonsb. Inherit the template in your custom modulec. Add a new div inside the template and call the salesperson from the objectd. Run the module
2)In each template layout there is a python function to get the contact information. You can override that function and add a salesperson to it.
example:

from odoo.addons.l10n_din5008.models.account_move import AccountMove
from odoo import models, _
from odoo.tools import format_date


class AccountMove(models.Model):
    _inherit = 'account.move'

    def _compute_l10n_din5008_template_data(self):
        for record in self:
            record.l10n_din5008_template_data = data = []
            if record.name:
                data.append((_("Invoice No."), record.name))
            if record.invoice_date:
                data.append((_("Invoice Date"),
                             format_date(self.env, record.invoice_date)))
            if record.invoice_date_due:
                data.append((_("Due Date"),
                             format_date(self.env, record.invoice_date_due)))
            if record.delivery_date:
                data.append((_("Delivery Date"),
                             format_date(self.env, record.delivery_date)))
            if record.invoice_origin:
                data.append((_("Sales Order"), record.invoice_origin))
            if record.ref:
                data.append((_("Customer Reference"), record.ref))
            if record.invoice_user_id:
                data.append((_("Salesperson"), record.invoice_user_id.name))

    AccountMove._compute_l10n_din5008_template_data = _compute_l10n_din5008_template_data

The above code is an example of a function in a specific localization template. So instead of it you can find the function of your localization and add similar things.

Regards