[FIX] account: send invoice mail in partner language

Steps to reproduce:

  - Install `account` module
  - Activate another language (e.g. French)
  - Create a partner with the new activated language
  - Create an invoice for this partner and sent it by mail

Issue:

  Some words in the external layout of the email are not translated;
  button and title

Cause:

  The language of the partner is not taken into account (except for the
  model name in the button) when creating the external layout of the
  mail.

Solution:

  Call send_mail with the partner language in the context.

opw-3301607

closes odoo/odoo#133744

Signed-off-by: William André (wan) <wan@odoo.com>