Yes, the international community should require Russia to clean the site because the pollution could spread and ruin other parts of the country. The world is not a place for humans to pollute. It is meant to be cared for and protected. Therefore, it is a tragedy that any place becomes polluted.
There should not be large amounts of radioactive waste just sitting around in one area. That is disgusting and hazardous to the health of any surrounding communities. The international community should require Russia to clean the site and they should be fined for allowing and approving something so dangerous to happen.
I would love for Russia to clean it up, for the sake of their own people and their own country. But other nations are not in the business of forcing this type of behavior. If there is evidence that it is so toxic that it is harming other countries around Russia that would be a different story. Without that evidence it is Russia's business.
It's hard for the international community to tell one country to clean up one specific thing, when every country has its pollution and things that it needs to improve. Also, pollution is the least of Russia's problems. If the international community is going to pressure Russia, it should be for human rights abuses and other acts against humanity.