Opacity transition on hover for my flag

I’m having issue while trying to add a transition for my flag icon. I can not find a way to add an opacity delay when hover on the french flag. Could someone help me with this please?
I know that this is a really simple thing but I just can’t find it and I’m starting to be mad
Thanks by advance for your time
Have all a nice day!

Here is my site Read-Only: [LINK][1]