There's no such thing as bad publicity. That's what Dries Tack, a Brussels-based marketing director, told himself when he found out that Republican presidential candidate and real-estate magnate Donald Trump took a dig at his home town on U.S. television last week.

Speaking on Fox Business Network, Mr. Trump called the Belgian and European Union capital, which is also home to several of the terrorists that attacked Paris in November, a "hellhole," where “something is going on, and it’s not good.”

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