2000 Volkswagen Golf Coolant flange

Cracks on radiators are non repairable, at least the repair won't last long as the material used is fiber based. Usually when this occurs, other parts of the radiator are also in poor condition.

A temporary fix would be to clean and dry the leaking area thoroughly and use a suitable epoxy sealer to patch the leak. A piece of masking tape over the sealer while it is curing would help to strengthen and hopefully you are able to make it last till you are able to get it replaced.

The radiator is subjected to heat so it would expand and contract making the repair unstable. You need to check the coolant regularly and ensure the area is not leaking. Be wary of other areas leaking.