With the Russian ruble in a nosedive under the pressure of Western sanctions and slumping oil prices, the country’s central bank decided Monday to freely float the currency in markets and stop regularly spending billions in a vain attempt to stem its fall. The bank has been burning through its reserves, which plunged from $510 billion at the year’s start to about $400 billion now, to soften the drop in the ruble, which has lost about half its value since the beginning of the year as investors pulled money out of Russia and the economy headed toward recession. It spent...