Bulgaria became the 21st country to switch to the euro as it entered the New Year on Thursday, a milestone met with both cheers and fears, nearly 20 years after the Balkan nation joined the European Union. At midnight, Bulgaria gave up the lev currency, which has been in use since the late 19th century, and Bulgarian euro coins were projected onto the central bank’s building. Successive governments in the country of 6.4 million people have advocated joining the euro, hoping that it will boost...