So do all office staff/engineers at the airline speak French, Dutch AND German.
Most of them, irrespective of whether they belong to the Francophone, Flemish or German communities, will speak their mother tongue and at least one other national language.
There is no escaping the fact that life here would be very difficult without a good working knowledge of French or Flemish and thus it is reasonable to expect an employer or anyone else for that matter to expect an applicant to be in a position to fully integrate.
This is not a protectionist issue - just common sense.