Can a semisweet chocolate be substituted with a belgian chocolate?
The two labels are referring to entirely different aspects of the chocolate, and are completely unrelated.

Semi-sweet refers to the approximate amount of cacoa solids versus sugar in the chocolate.

Belgian refers to the location of the processing plant where the cocao beans were processed into chocolate.

The label for your chocolate should indicate a percentage of cocao solids, or a label like "semi-sweet" or "bitter-sweet" although these terms are not very tightly or uniformly regulated around the world.

Most brownie recipes are going to be very tolerant of the particular chocolate used, so the substitution should work well, especially if your Belgian chocolate happens to be in the approximately semi-sweet range.