Taking a saying from a language you do not speak and trying to "translate" it is hard enough, given the challenging nature of idiomatic words and expressions. Coming up with an equivalent saying in a language you do speak often compounds the difficulty.
Frankly, other contributors to this post have beaten me to the punch with "20/20 hindsight," "Too little, too late," and "A day late and a dollar short."
While this may not be an answer to your question, it is somewhat apropos nevertheless. The French, I am told, have a saying that could be translated into English, roughly, as "brilliance on the staircase"; that is, thinking of the right thing to say when it's too late to do any good. Say some smart aleck at a party (or wherever) aims a snide comment at you. You are temporarily rendered speechless or you blurt out something feckless like "Oh yeah?!" Then on the steps as you leave the party you think of the perfect riposte, but it's too late. (D'oh! as you smack your forehead). Is there an English equivalent?
Well, how about "That ship has sailed"? Or, "Monday morning quarterback?" Or, "We get too soon old, and too late smart"? Or, "Why didn't I think of that!?"? Or, "You can't reinvent the wheel"? Or, "You can't un-fire a gun"? Or, "You can't un-ring a bell"? Or, "That's water over the dam"? Or, "Beating a dead horse"? Each one of the foregoing has a flaw or two, I suppose. Any other suggestions out there?