Belgium
Belgium is the most complex of the three, as it is not one unified system. The country is split into three linguistic communities:
Flemish Community (Dutch-speaking, e.g., KU Leuven, Ghent University)
French Community (French-speaking, e.g., UCLouvain, ULB)
German Community (very small)
You are applying to a university in one of these communities, and the rules are completely different for each. There is no central application portal.
Language: different language depends on the Univ
HS certificates need to be as equivalent as a Belgian diploma (this is the hardest part) and the official letter from NARIC or Service des Équivalences.
CV and essay