Hikaerial : change date format in attachserial

-- url of the page with the problem -- : www.bapteme.air.tousenvol.com
-- HikaShop version -- : 2.4.0
-- HikaSerial version -- : 1.9.1
-- Joomla version -- : 3.4.1
-- PHP version -- : 5.6

Hello,

How could i change the printed date format in attachserial plugin ?

At the moment Hikaserial display the order date in US format : Thursday, 23 April
I would like to get in as : %d-%m-%Y

I added this line in override language section in Hikashop, but this did not help :
HIKASHOP_DATE_FORMAT="%d-%m-%Y"

Many thanks foryour help,