conuly

The "One Belgian" is presumably Jewish. As Wikipedia says, Jews are a small minority in Belgium - some 50,000 people out of a population of 10.5 million. (It is apparently half the population that there was prior to the Holocaust, but many of those seem to have been refugees from Germany.)

So the way to identify as Belgian instead of dividing yourself into Flemish or Walloon would seem to be outside the normal system of dividing the population entirely. Of course, all the Flemish and Walloon soldiers are Belgian too. Alternatively, this soldier doesn't identify himself as Belgian but as Jewish first, it's just that "Jewish" wasn't given as an option.

Comments to the entry I linked to say that a common joke among Belgian Jews is that the only real Belgians are Jews and the King.