[Faceman]

The Austrian "Verbotsgesetz 1948" of course which makes punishable by law
1. the forming of a group intending to "resurrect" the NSDAP
2. publication of Nazi propaganda
3. denial of the holocaust in public
etc.

This is why in Austria it is actually illegal to own a private copy of "Mein Kampf" for example (of course still openly available in libraries).

From a judicial POV I doubt that denying the holocaust in private conversation is illegal in Belgium, but rather that like in Austria it is punishable to state it publicly or publicate it in written/recorded form.