The Constitutional Court of Belgium on June 26 annulled an article of the alien law of 2006 that gave no right of family reunification to children born of a polygamous marriage. Now an alien living in Belgium can request that children from a second marriage, probably living in his home country, can legally immigrate to join him. (See this Islam in Europe entry.)
So is polygamy itself now legal in Belgium? I'm not a lawyer, so I leave that question to others, but I suspect that if it isn't, it soon will be.