[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#121086

X-original-commit: f09ac453aa7cffe1e31f30b68fa0665ef504283a
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: Mahdi Cheikh Rouhou (macr) <macr@odoo.com>

addons/loyalty/data/mail_template_data.xml

@@ -5,6 +5,7 @@
         <field name="model_id" ref="model_loyalty_card"/>
         <field name="subject">Your Gift Card at {{ object.company_id.name }}</field>
         <field name="partner_to">{{ object._get_mail_partner().id }}</field>
+        <field name="lang">{{ object._get_mail_partner().lang }}</field>
         <field name="description">Sent to customer who purchased a gift card</field>
         <field name="body_html" type="html">
             <div style="background: #ffffff">