ATHENS — The inglorious old French expression “les villes du sud puent,” meaning, “cities of the south stink,” may need a petite modification. Parisians in particular have traditionally turned their noses up at any major conurbation south of the Seine, but it is looking more and more like oddball Brussels is the most malodorous city of all. The fumes, which emanate from a variety of sources, cast doubt on the credibility of the European project even as it flirts with expansion.