If we're being pedantic, the US is not a nation-state (because the US doesn't really have a coherent "nation"), and I suspect for similar reasons you could claim that Belgium is not a coherent nation either, given the Flemish/Walloon divisions in the state.
That said, a lot of people use nation state as a synonym for sovereign state, and Belgium is as much a sovereign state as the US is.
That said, a lot of people use nation state as a synonym for sovereign state, and Belgium is as much a sovereign state as the US is.