Why is that a problem? We can still understand all the writings on ancient Roman buildings without any trouble. And it isn't too hard to make pictograms showing that something is dangerous and radioactive. We put some effort into universal pictograms understandable by actual aliens for the Voyager probes; making pictographs understandable by humans is much simpler.
Finally, why are you so determined to believe that no one will have a solution for radioactive waste in a few hundred years? How hard will it be to launch it off the planet if necessary? With the cost/kg of rocket launches coming down, it shouldn't be too hard to dispose of it that way. But for some reason, people like you just insist on sticking with coal power and breathing in all the pollution that creates. I will never understand it.