Why is French spoken in Belgium?

Once colonized by the Germans, during WW1 the British and French divided the country after the war. Four fifths of the country was assigned to the French under the League of Nations treaty. From 1919 to 1939 the French build schools , roads and industries and the working language was French.
People in Cameroon speak French because Cameroon was a French colony from 1919 until 1959.
Cameroon was a French colony, ruled by France, for many years.