The Dutch word gezellig
is one of the most difficult words to translate, but you'll know it when you
feel it: a gezellig environment is one which allows good times
to happen. Gezellig is comfortable, but also lively: it goes deeper than mere coziness. It's all about a sense of belonging--in this way, it's a near antonym to another untranslatable word, the Portugese saudade, which connotes an existential homesickness.
