Is Belgium unilingual?

No. In the region of Flanders (North) people speak Dutch, in most of Wallonia (South), people speak French and in the very east of Wallonia there are a few towns (Eupen, Malmedy, Sanct-Vith) where they speak German. The capital, Brussels, has two official languages, French and Dutch, but many other languages are spoken: English, Spannish, Portuguese, Arabic ...