Some governments (like El Salvador and Russia) are moving into bitcoin mining.
Do you mean the Russians here or the Russian government, because I don't think I've learnt of the Russian government moving into Bitcoin mining of recent, but I know well that of El-Salvador, they have been into bitcoin mining and investments mostly over the years.
If more nation-states accumulate BTC, what impact will this have on decentralization and price stability? Could this lead to geopolitical bitcoin wars?
No, Bitcoin cannot be centralized at this cause, the market cannot be manipulated in their behalf for this reason, they can't be the only whales that will be investing on bitcoin, just as we have it now, no whales have been able to achieve any form of centralization over bitcoin.