[REF] sale: improve usage of composer mixin in order cancel

We can call ``_render_field`` on template model directly. In this commit
we also add the dynamic language computation through ``_compute_lang``.
It will use the field defined in template.

Task-2710804 (Mail: Clean MailThread API)

Part-of: odoo/odoo#106072

addons/sale/wizard/sale_order_cancel.py

@@ -60,26 +60,23 @@ class SaleOrderCancel(models.TransientModel):
 
     @api.depends('order_id')
     def _compute_subject(self):
-        for wizard in self:
-            if wizard.template_id:
-                wizard.subject = self.sudo()._render_template(
-                    wizard.template_id.subject,
-                    'sale.order',
-                    [wizard.order_id.id],
-                    options={'post_process': True},
-                )[wizard.order_id.id]
+        for wizard_su in self.filtered('template_id').sudo():
+            wizard_su.subject = wizard_su.template_id._render_field(
+                'subject',
+                [wizard_su.order_id.id],
+                compute_lang=True,
+                options={'post_process': True},
+            )[wizard_su.order_id.id]
 
     @api.depends('order_id')
     def _compute_body(self):
-        for wizard in self:
-            if wizard.template_id:
-                wizard.body = self.sudo()._render_template(
-                    wizard.template_id.body_html,
-                    'sale.order',
-                    [wizard.order_id.id],
-                    options={'post_process': True},
-                    engine='qweb',
-                )[wizard.order_id.id]
+        for wizard_su in self.filtered('template_id').sudo():
+            wizard_su.body = wizard_su.template_id._render_field(
+                'body_html',
+                [wizard_su.order_id.id],
+                compute_lang=True,
+                options={'post_process': True},
+            )[wizard_su.order_id.id]
 
     def action_send_mail_and_cancel(self):
         self.ensure_one()