gezellig is one of the most well known untranslatable dutch words. when you plug it into google translate you'll get a whole bunch of words that, on their own, at best approximate what gezellig is as a concept, but no single word fully captures ALL of it. gezellig is just one of those very very fuzzy concepts that's kind of impossible to describe but gets used to describe a pretty wide amount of things.
but also i just remembered that the archaic usage of the word "gay" probably actually gets the closest, to where i'd say it is a direct translation of the word gezellig. having a gay old time at the picnic. having a gay evening at the bar. going to this new part of the city for the gay atmosphere. it fits so well