I mean that there is no Belgian nation - native Belgian citizens are mostly either Dutch-speaking Flemish people or French-speaking Wallons, and generally don't have any significant notion of a shared national identity.
Note that this doesn't mean that a state with multiple ethnicities/languages can't be a nation state. Indians, for example, generally have a clear national identity, despite being citizens of a huge federal republic with dozens if not hundreds of languages spoken, some of which don't even share a common language family. So, India is a nation state, unlike Belgium.
I don't know if i agree that Belgians don't have a national identity. I just think that the Flemish nationalist parties push the Flemish nationality identity so hard that Flemish people more identify as Flemish then as Belgian.
However Walloon people definitely feel Belgian and have a Belgian pride.
Also, i don't know if a nation state is defined by having a national identity?
Note that this doesn't mean that a state with multiple ethnicities/languages can't be a nation state. Indians, for example, generally have a clear national identity, despite being citizens of a huge federal republic with dozens if not hundreds of languages spoken, some of which don't even share a common language family. So, India is a nation state, unlike Belgium.