The three-word message, launched minutes after the verdict came in, was succinct in its solidarity. “Je suis Marine!” Hungary’s Viktor Orbán posted on social media after France’s far-right leader, Marine Le Pen, was found guilty of embezzling European parliament funds and barred immediately from running for public office. Messages soon came tumbling in from Brazil to Belgium, hinting at how rightwing nationalist and populist leaders had seized on the ruling to push their own narrative. Most of them paid little heed to the judges’ finding of the key role Le Pen and more than two dozen others had played in...