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.
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?