[FIX] loyalty : send email in partner language

When sending email with gift card, it will have the user langauge and not the partner language

Steps to reproduce the error :
1- Add french and english language
2- Make the current language english
3- Install contact and sales and activate gift cards
4- create a contact having french language
5- create a gift card for the french partner and send it to him
6- the email will be in english lang

The origin of the problem was the missing lang field in the template

opw-3308919

closes odoo/odoo#121028

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>