I remember hearing eudaimonia many years ago and thought that I can't wait for it. I haven't heard it since then and the state of the world reminds me I may never again.
Yeah désuet and flâneur are as common in French as abandoned and loiterer are in English. I'd say only the first word describes something that there isn't another common word for.
I knew flaneur as an adjective in German, for example when you go on a walk through a shopping street, without any intent of shopping you go "flanieren". Just having a walk, looking at stores and people.
