Brussels it is, perhaps because, geographically, the beautiful City is at the crossroads, being within 1,000 km. (~400+ miles) of most western European capitals.
And Belgium has 3 official languages: Dutch (spoken by about 60%), French (spoken by about 40%), and German.
It helps that NATO has its headquarters in Brussels and most of the UN's organs in Europe are based in that City!
Get this. Although Brussels is geographically situated in the northern portion of Belgium (traditionally of the Dutch-speaking Flemings), French is the "lingua franca" in the City!
(Vive le France!)