I didn't see anything in the article stating that anyone had plans to sue.
But if they do, I suppose we can just tell them to deduct it from the $1.7 trillion that WWII cost us, that we haven't gotten around to billing them yet.
I doubt they'll be able to sue anyone but Germany until they get the numbers right, the allies never dropped a single 1,100 pound bomb on Europe.

But seriously, this is a very tragic event in which 3 more inocent lives were ruined by a horrid war long passed.
I do think it is rediculous to try and seek legal retribution on this and would hope that they instead focus on what to do when they find the next one. As someone mentioned, the fuses are very old and likely extremely delicate in their current condition if they could still somehow function at all. Also to consider is the stability of the high explosives after being entoombed that long, especialy if it's dynamite-based (aged nitro-glycerine based explosives can be a nightmare to handle if the stabilizer used in it degrades over time, best example of this is dynamite itself, it's nitro-soaked sawdust... once the sawdust starts to decompose you're left with mostly raw impact-sensitive nitro... the type of stuff that detonates if a feather drops on it at the right angle.)