After a marriage, the city prepares an official document, a marriage certificate. You can request a copy of it online.
Do you want to get married in Leuven? Then you will have to file a declaration at the city office.
2 people living together can make a declaration of registered partnership (legal cohabiting contract) at the city office.
If you get married abroad you must have the marriage recognised in Belgium.
Professor Van Overstraetenplein 1 3000 Leuven