Did you work in Belgium and were your employment rights not respected? If so, you can still enforce your rights even if you are no longer in Belgium or are planning to leave Belgium. It is best to take action as soon as possible. It is obviously better to take the necessary action before you leave Belgium . Whether your case is likely to succeed is dependent on the documentary evidence you have.
If you are no longer in Belgium, someone else will have to monitor your case for you; this could be lawyers, your trade union or FAIRWORK Belgium. They will need to stay in contact with you during the procedure. So make sure they always have the right contact information for you, wherever you are in the world.