Smug is an adjective, according to my Merriam-Webster´s, apt to denote a person
having or showing the annoying quality of people who feel very pleased or satisfied with their abilities, achievements, etc.
I reckon the key word in this definition is annoying. It is easy to express in German or Spanish that somebody is obviously very pleased with him- or herself. In Spanish one could say: autosatisfecho, encantado de sí mismo, pagado de sí mismo, no tiene abuela... In German you have selbstzufrieden, selbstverliebt, perhaps even selbstgefällig. Some online dictionaries even suggest blasiert or dünkelhaft. But I reckon none of it is exactly the same. And although it is evident that those words are not meant to flatter I still miss the aspect of being annoying that the above definition so clearly states.
So what is to be done? Perhaps we could consider applying violence. It does not solve the problem of translation but it gives the smuggies you do not like what they deserve. Not the best solution, I admit, but it´s a start.