PRAGUE—Authorities in central Europe warned on Friday that they are seeing a surge in coronavirus cases across their region amid the discovery of new variants. The Polish health minister, Adam Niedzielski, said the first case of a variant first found in South Africa had been discovered in Poland, and that some 10 percent of all cases now involve the variant that emerged in England. “We are entering a crisis situation again. It is up to us where the peak of this third wave will be” Niedzielski said. “Responsible behavior is key.” The Czech Republic, one of the hardest-hit countries in the 27-member European Union, moved Friday to further tighten restrictive measures amid a surge of the English variant. The government said the worsening situation has forced it to abandon its plans to reopen all stores as early as next week. Neighboring Slovakia is also badly hit. It became the country …