Can I use belgian chocolate as a substitute for semisweet chocolate for baking brownies?
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.
Sure, you can make that substitution. On the whole the better quality chocolate you put into your brownies the better they will taste. The thing you need to pay attention to is sugar. If you substitute a sweeter chocolate you'll want to reduce the sugar added somewhat, if you substitute 70% dark for milk chocolate you'll want to add a bit of sugar. Taste the batter to get a feel for the sugar content.