I'm quite sure that capital gains taxation in Belgium can only concern capital gains made on real estate located in Belgium and never the capital gains made on private real estate outside Belgium. Capital gains tax on private real esate is a very limited tax in Belgium. We have no general concept to tax capital gains. Capital is taxed in other ways.
Rent, interest, profit, wage and divided earned worldwide, on the contrary are in principle taxable in Belgian income tax taxation (except when doubble tax treaties are applicable).
Even when somebody isn't actually living in the South of France, I know for sure that there is a complicated system of capital gains taxation by the French Government on real esate in France. This tax is also applicale to foreigners who sell their real esate located in France. So, there propably will have to be payed something, when the deed of sale is passed before the notary in France. The notary will have to deduct it from the sale price, and pay it directly to the French tax office.
In Belgium and France real esate transaction must be done by the office of a notary. These a very competent lawyers. You need a notary anyway when you make a real estate transaction in Belgium or France. A notary can advise you exactly about the capital gains tax if you would sell.
If you live in Belgium, you have to make a yearly income tax declaration here. In that declaration you would have to mention eventual capital gains which are taxable (resale within 8 years), but when it's your private residence, you won't have yo pay capital gains tax).
If you are living abroad, (the worldwide headquaters of you private fortune isn't located in Belgium), you still have to pay capital gains tax in Belgium on the resale of real esate located in Belgium. In that case you can never sell your principal residence. It doesn't matter for the tax office in Belgium that you used the real esate as a your principal residence during the years before.
Still, if you owned the real esate in Belgium for over 8 years, there can never be capital gains taxation by the Belgian tax office and it then doesn't matter where you live at the moment of sale.
In Belgium the fee of the notary for a real estate transaction is fixed by law and is always payed for by the buyer. He can choose a notary. If the seller wants his own notary to advise him, both notaries will split the fee payed by the buyer.
This page is free general information to communicate the ideas. For exact legal advice, you should consult a professional and present him your case.