Question

What type of language are you using when you say you are pleased when you are really extremely happy?

Answer 1

You are making an understatement.

Answer 2

I would say understatement.
Example: You fall down the stairs and break your leg.While the ambulance is on the way, your friends asks how much it hurts and instead of wanting to seem like a crybaby you downplay how much pain you're actually in. Which is an understatement.