Why a vote on taxing California’s billionaires is being watched around the world

US independent senator Bernie Sanders at the launch of the campaign for the California Billionaire Tax Act. Patrick T. Fallon/AFP via Getty Images In November, California will vote on a proposal to levy a one-off 5% tax on the wealth of all billionaires in the state. Google co-founder and billionaire Sergey Brin has already spent US$100 million on political donations to a group opposing the tax. Brin is hoping to avoid a potential tax bill of US$13 billion, a return of roughly 130 to 1 if that…