I got the french edition of fallout but i bought add-ons worth 20 euros. These add-ons arent compatible with the french version of the game so would it be possible to trade the german add-ons with french ones ?

seriously doubt it. im sure bethesda will say to you "Here at Bethesda studios, we are sorry that you made a mistake. Since we already have your money, we're not interested in helping you any further. Thank you" If I were you I'd just start learning Germen (or French, I cant really tell what your talking about) sorry mate

No company would give you a refund/trade through PSN

if you were to ask anyone for some sort of refund or even swap so you can get the right DLCs is sony you want to ask really not bethesda, because you bought it from sonys site however im not sure they will do that for you but tis always worth a try