Je ne sais pas sur quel pied danser...I don't know which foot to dance on.
Which actually means to not know how to react or which side to take, which decision to make with regards to someone or something.
I like it in French - it sounds just a little bit whimsical. And of course it calls up an image. French is good for that, colorful expressions that easily lend themselves to images. There's actually an adjective that means just that - imagé - colorful but not in the full-of-color way. You know what I mean.
Anyway, back to dancing on one foot. Or the other. I've been doing it lately. And while I love to dance, I'd like to stop.