When looked at that way, with the implication being that he is dragging a heavy weight around (hence zuru zuru) the line about him "losing some weight" or the sub-titled version I always saw "I think my burden has lightened" makes a lot more sense outright, what with the dragging metaphor. But yes, I agree they had to replace it with something, since not many would actually know this outside of Japan realistically. So take your pick, something silly, or something more literal like the "Dragging... dragging... always dragging!"
As for Reno seeing what Tifa meant, well if she was basically stating that Cloud is dragging too much weight around, he can either think Cloud needs to lose weight, get a smaller freaking sword, or understand its a metaphor for emotional baggage weighing him down, right?
For the record, since I prefer sub to dub any day, when I saw Cloud dragging the Buster Sword away from Zack's corpse at the end of CC, I found myself sadly saying "zuru zuru, zuru zuru" cause come on? So symbolic.