You'd think that a skull-and-crossbones symbol would strike fear into people no matter where they are, making it a good warning symbol for danger in all languages. But over time, the Jolly Roger came to be associated with pirates and treasure maps, so not only is the fear muted, the symbol became attractive. Hey, there may be treasure buried here! What seemed to be common sense became useless to warn people away from dangerous places, like, say, a nuclear waste disposal site. So how do we label such dangers in a way that people who don't read, or don't know the language, will know what they mean thousands of years in the future?
Designing a warning sign is much more complicated than you'd think. In addition to this video from Vox, you can read the accompanying article at 99% Invisible.
See also: How the Government Plans to Protect Our Nuclear Waste.