After editing full core.yml, I still wasn't able to edit "3 days ago" and others like that. Where are they? :X
Where to find time language files?
Moment.js handles all that. I'm not too sure how that is added, best bet is checking one of the more used language extensions, eg:
https://github.com/Kakifrucht/flarum-de/blob/master/locale/config.js
Helpful as always, thanks! I will look into that. ?