"You seem happy today."
Translation:Du verkar glad i dag.
Good question! It's hard to translate lycklig directly. While they're certainly in the same ballpark, I doubt many Swedes would say that the differences are subtle.
Think of it as the difference between being satisfied/content and being happy, but kick it up a notch. That's lycklig. You're more than happy - you're truly, really happy, and the mood is hard to spoil by bad news. It might be because of current circumstances, such as your new relationship being awesome or your promotion came through. Or it might just be that you're high on life.