I thought of a better translation that works in all casual situations our even at work. "Great job" But bottom line there is not one single translation that works in English. We have many different based on the situation. Whereas 辛苦is much more versatile. Very good question. "Great job!" It's just a way for saying great job and also may be thanking them, but not necessarily if you are not benefit from the work. So also it can just be an expression to say "you're working hard" I'll keep my original response below too: "Appreciate your hard work", "thanks for your hard work" when you say 辛苦 , your recognize the other persons hard work and showing your appreciation for it. So in English most naturally I would say "thanks for your hard work" or "thanks, I really appreciate your hard work"

我感觉，汉语语境里，“辛苦了” 有两种情况。 1， 别人为你付出了劳动，比如帮你搬家，那么你说 辛苦了，相当于 thank you so much, thanks a lot. 2， 别人付出劳动，但不是为你。比如人家照料生病的家人，你看到了，你对他说 “辛苦(你)了”，是在鼓励、安慰他。相当于什么呢...我估计是 I know it's hard for you, keep up the good work.

I disagree with the answer below. Honestly, I'd usually translate it as "thanks a lot".

"You poor thing".