You are correct:
Captain's side (left): correct order
Co-pilot's side (right): backward order
This because a flag on a "vessel" (ship, plane) is seen floating in the wind with the nose/bow being the flag mast. Therefor the left side of the plane has the flag in correct order while the right side has it in reverse order. Or with the Belgian flag: black, yellow, red seen from the left and red, yellow, black seen from the right.
According to the pictures of OO-ABB the flag besides the registration is placed incorrect (at least on the right side). Unfortunately a flag or the correct depiction of a flag besides the registration isn't mandatory by law (in Belgium, in other countries it is).
Yeah, I know, i'm an annoying guy that keeps bringing it up But if you have the audacity to call your airline "Air Belgium" then the least one could do is get the flag right.