If Decathlon charged you for an item but didn't confirm the order, log in to your account and check "My Orders" to see if the charge is there.
Their help page says that if the payment is taken out but you don't hear anything within 90 minutes, you will automatically get a refund, which will be credited back to the way of payment within 72 hours.
If that time has passed, call Decathlon's customer service and give them your order number, the reference number for the payment, and a copy of your bank account that shows the deduction.
If that doesn't work, you can ask your bank to challenge the charge as an uncompleted transaction.
If none of the above solutions work, please feel free to share your experience. You can also file a complaint on our website.