MOSCOW—Prison doctors have decided to transfer Russia’s main opposition leader Alexei Navalny to hospital, its prison authority said on Monday, 20 days into a hunger strike that has brought international warnings of consequences should he die in jail. Allies of Navalny, who have had no access to him since last week, said they were braced for bad news about his health. They are planning mass countrywide demonstrations later this week. Navalny’s case has further isolated Moscow at a time when U.S. President Joe Biden’s administration has announced tougher economic sanctions and the Czech Republic, an EU and NATO member, has expelled Russian spies, accusing Moscow of a role in deadly 2014 explosions at an arms storage depot. Russia’s prison service said in a statement that a decision had been taken to transfer Navalny, 44, to a regional prison hospital, although it did not make clear whether the transfer had already …