Do you think Russia and China are doing this because they are expecting an economic crisis on the horizon? This might sound paranoid but maybe it's possible that they will both try to cause the crisis in order to profit from their gold investment.
Perhaps they are doing this to insulate themselves from any crash in the value of the US Dollar. Gold is one of the few assets which are protected against currency devaluation. The other assets such as equities and bonds lose their value along with the currency.
The reserves in a foreign currency usually tend to roughly match the national debt of the state in that currency. In this way, the crash in the value of the US Dollar would not only devalue these reserves but also devalue the debt itself. So it is effectively six of one and half a dozen of the other. On the other hand, if the national debt tends to diminish over time (as is the case with Russia), it doesn't make much sense to keep the reserves of the currency any longer...
And buying gold with them looks a better idea on the whole