US is China's largest trading partner. Every year, US is running deficit of $300-400B. Foreign Trade - U.S. Trade with China (census.gov) Since, the trade is in US$, China needs to park its gain somewhere. China can convert US$ to RMB but that it would drive up the RMB and undermine China's competitiveness. It's convenient way to park it's asset. If I am not wrong, China did try on Euros. Certainly, not ₽. Unless, China and US choose not to trade, this is going to be an ongoing thing.
Maybe they can park some RMB's in that super, super valuable Russian Ruble that wingnuts are all excited about these days. lol