Americans crossing their fingers in movies when they’re hopeful is always confusing since here that means you’re lying. 

It means that in america too, it just depends on the position. 

Crossing fingers behind your back when making a statement means you are lying.

crossing them in front of your body is hoping for some good luck

I see! We just don’t have the good luck thing.

Over here you express the same thing by holding your thumbs. We say “I’m holding my thumbs for you”: