qgis - How to display attributes of features intersecting the Atlas feature in a Print Composer label?

Is it possible to edit labels with python or an expression?

I have an atlas layer and a shape with countries. the atlas layer has several features and each feature intersects with several countries.

I want a label on my pdf map that gets the country name (attribute in country layer) as a function of the intersection of the atlas layer. is something like this possible?

Example: Feature 1 of Atlas Layer lays over 3 Countries (C1, C2, C3). I want a label on my map that says: "C1, C2, C3". Feature 1 of Atlas Layer lays over 2 Countries (C5, C6). I want a label on my map that says: "C5, C6".