I'm sorry, The_FD, but I'm a french belgian and I never heard anyone saying 'octante' instead of 'quatre-vingt', but I think french swiss people maybe say it...
There're some 'belgicism' dictionaries on the net... but they are in french, it couldn't be easy for Yau now but The_FD can explain him it!
http://membres.lycos.fr/clo7/histoire/belgique.htm take a lot of belgicism examples and
http://atelier5.webacademie.ch/scrab...elgicismes.htm is a very complete dictionary of belgian words.