First little research I did, gives;
"Van links naar rechts zien we de enige zoon van Baron Henri de Fierlant : Adriën de Fierlant , in zijn uniform van luitenant 2e. Lansiers , die omkwam in het jaar 1908 in het Congolese LUBA gebied als Agent voor de Belgische firma LOMAMI ."
Meaning;
From left to right we see the only son of Baron Henri de Fierlant: Adriën de Fierlant, in his uniform of lieutenant 2e. Lancers, who died in the year 1908 in the Congolese LUBA area as Agent for the Belgian company LOMAMI.
This info was kindly given by the history society of Kampenhout (the city where this family used to live). And they apparently have the same picture, according to them taken;
"Foto genomen in het jaar 1900 door de Brusselse hof-fotograaf - Ferd. Buyle - 104 Marché aux herbes Bruxelles "
Meaning;
Photograph taken in the year 1900 by the Brussels court-photographer - Ferd. Buyle - 104 Marché aux herbes Bruxelles
Just a little mistake (to me!, when they say the guy is wearing a lieutenant uniform...he might have become officer in a later stage, but here the uniform shows the rank of the highest NCO (a wachtmeester).