There has been much thought given to designing signs that would signify the presence of a nuclear waste dump to a reader with no cultural connection to the dumper.
I propose a very simple solution. The only thing that reliably signifies radiation in a way that is not dependent on any cultural or linguistic context is nuclear radiation itself. Hence, one should not simply have an isolated nuclear dump surrounded by pristine material, such that one that missed or misinterpreted the sign might dig unsuspectingly, and dig some more, and then suddenly hit upon something rather toxic.
Instead, the degree of radioactivity should be ramped down gradually, such that the general area of the dump would be sufficiently radioactive to make an observer with a Geiger counter mildly surprised, and as he gets closer and closer in his digging, increasingly alarmed. Regardless of cultural context, if radiation levels steadily increase as some point is approached, an intelligent observer will at least suspect that they will continue to increase as the point is further approached.
You may counter that the future civilization getting the message may have entirely lost the concept of nuclear radiation, Geiger counters, or anything like that. But in that case, you're stuck anyway. There is no way that a sign that merely conveys "danger," without being more specific, is going to scare people away. Nothing that the ancient Egyptians could have written on their pyramids in any language would have kept people from being intrigued by what could be inside them.
In particular, Edvard Munch's painting The Scream, which I believe has been proposed as a nuclear warning device, probably does universally signify fear, at least to humans that share our innate emotionally determined facial expressions. But that would only make the disposal site more interesting to future archeologists, not less.
