Adding dropdown to a list add buttom

I'm working on a custom inspector and, right now, I'm trying to find a way to add this behavior to a list on the custom Inspector:

Basically, the + sign button on a list gets a small triangle so that, when the button is clicked, instead of just adding a new empty element to the bottom of the list, it displays a list of options to choose from. Each option from the dropdown adds to the list a custom value when clicked.

(For a practical example, go to Edit > Project Settings > Player > Other Settings and see the Color gamut for Mac List).

The main issue is just: how to display a list in this way? Is there a function on the EditorGUILayout, EditorWindow, or something that can help me achieve that?

Important! I'm not trying to create something totally new or to mimic this behavior using cleverly positioned buttons with custom icons. I remember implementing this a while back but I just can't find it anymore no matter how hard I search on the internet.

Context

I'm building the custom inspector for the list of available languages for a localization tool. What I want is the dropdown to display the available languages that can be added. To do that, I need the list to have this dropdown behavior. (Right now the LocaleDictionaries is built as a Scriptable Object).