Skull and Crossbones has been a symbol associated with death at least since Danse Macabre in the 1400's. By the late 1600's it began being a flag used by pirates when they were raiding. By the 1880's the skull and crossbones was the ubiquitous warning of poison.
It would be best to bury multiple warning symbols as these are likely to outlast our languages. If you see a skull and cross bone symbol then you're going to at least pause for thought. Bury radiological, biohazard, etc. warnings just in case one has survived in some form of recognition, at least someone might stop and think "hey are these symbols in our archives anywhere?"
Then I would ensure writing is buried at a deeper level. Place warnings in every major language. Lexicologists(?) are anal retentive, they've managed to keep Latin and Ancient Greek around despite not a single native speaker. It's safe to assume that languages can last a long time, it's just a question of which one.