They are not likely to use actual bitcoin, more likely just a blockchain implementation to reduce their costs.
I would even argue it may be irresponsible to run a banking system on the public chain.
The current system of international money transfer through SWIFT is too much of a hassle. Sometimes the payments gets delayed by up to a week, and on rare occasions, the transactions are lost without any trace. That is one of the reasons why the banks are experimenting with new technology such as Blockchain.