[FIX] fix lang in mail template. take the lang from the partner_id

instead of tax.shelter.certificat which do not have a lang field

easy_my_coop_taxshelter_report/data/mail_template_data.xml

@@ -10,7 +10,7 @@
             <field name="reply_to">${(object.company_id.coop_email_contact or object.user_id.email)|safe}</field>
             <field name="model_id" ref="model_tax_shelter_certificate"/>
             <field name="auto_delete" eval="True"/>
-            <field name="lang">${object.lang}</field>
+            <field name="lang">${object.partner_id.lang}</field>
             <field name="body_html"><![CDATA[
 <div style="font-family: 'Lucica Grande', Ubuntu, Arial, Verdana, sans-serif; font-size: 12px; color: rgb(34, 34, 34); background-color: #FFF; ">
     <p>Hello ${object.partner_id.name},</p>