LONDON — No doubt Donald Trump will be thrilled that the entire world is mesmerized as he steamrolls over his Republican rivals for the presidential nomination. He probably doesn’t care that most of the international publicity is bad publicity—but even the master of ill-tempered putdowns and Twitter vitriol will struggle to keep up with the sheer number of attacks and jibes pouring in from every corner of the globe. He does have a select band of fans—more on those later—but they are being drowned out by an incredulity that stretches from Europe’s capitals to post-conflict Afghanistan; from the African deltas...