If, at the beginning of your traineeship, you registered with a Commune and then deregister immediately after you finish your 5-month traineeship, you will receive a tax declaration form but – as a non-resident – you will not be required to pay any tax on your living allowance as your stay will be shorter than 183 days.
If, at the beginning of your traineeship, you registered with a Commune and, after the traineeship’s end, you extend your stay in Belgium beyond 183 days, you will become a tax resident in Belgium and your living allowance will be taxed. Note that the Belgian legislation allows for the deduction of certain costs, such as housing and public transport expenses (proof of these expenses will be necessary). This may significantly reduce your taxable basis and may result in you paying a small amount of tax (or no tax) on the living allowance you have received during your 5-month traineeship.