Just printing currency without consideration of your production output, raw material or manufactured goods alike, creates inflation that hurt yourself. The only exception is USD because US can export their inflation to the world because USD is a world trading currency.
If your own currency is accepted in international trade (such as energy trade between Russia and non western countries), it will serve as the same purpose as foreign reserves. Essentially Russia keeping Ruble reserve is the same as RMB reserve because the two currencies are agreed at an exchange rate and accepted in bilateral trade. This means there is no need for your bank reserve to be in a specific currency.
I think your thinking is based on the Dollar dominated world trade establishment. In that world, non-western currencies are not accepted, therefor you need reserves in western currencies. Here west = "foreign" is only because of US rule.
But in a parallel world without the west such as trade among China, Russia, India etc. there is no need for these "foreign" currencies. If it is not because of US military dominance and tie of ME oil with Dollar, there wouldn't have been a need for "foreign"(USD) currency reserve at all.