Which DLC I need to concentrate development in EU4?
I've seen in many youtube videos (e.g. this) that in the state view there is a button to concentrate development and make coring provinces cheaper. However, I cannot find that button when I play EU4. Which DLC do I need to enable it?
You need the latest big one - Leviathan. Here's the wiki page that lists all dlc and features they unlock
https://eu4.paradoxwikis.com/Downloadable_content
Also notice that you lose some portion of development. It's not just free cheaper cores. And the upcoming free patch is going to nerf that ability significantly