Is it permissible to train in Islamic banks – for a period of 3 or 6 months – and obtain a training certificate, even though I prefer not to work in them because they might deal with usury?

If the bank has a reliable Sharia (definition) supervisory board that oversees its contracts to clarify what is permissible and what is forbidden, then there is no harm in working for it, whether the work is permanent or temporary. However, if the bank is usurious, and the training involves writing usurious contracts, or bearing witness to them, or any form of assistance to the bank in its wrongdoing, then it is not permissible, because it is an aid to sin and aggression. But if the training is free from that, then there is no objection to it.