[FIX] website_sale_stock: custom message translatable

Make the 'Custom Message' field translatable since it is supposed to be
used on the website and displayed in the language of the user.

opw-2008088

closes odoo/odoo#34344

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/website_sale_stock/models/product_template.py

@@ -14,7 +14,7 @@ class ProductTemplate(models.Model):
         ('custom', 'Show product-specific notifications'),
     ], string='Inventory Availability', help='Adds an inventory availability status on the web product page.', default='never')
     available_threshold = fields.Float(string='Availability Threshold', default=5.0)
-    custom_message = fields.Text(string='Custom Message', default='')
+    custom_message = fields.Text(string='Custom Message', default='', translate=True)
 
     @api.multi
     def _get_combination_info(self, combination=False, product_id=False, add_qty=1, pricelist=False, reference_product=False, only_template=False):