I'm not sure about most banks, but some use other industry-specific software, there's one being developed in Uruguay for small banks and financial companies called Bantotal, which is written in Genexus, a 4GL language or metalanguage that generates whichever language you want (Java, Ruby, .NET, even Cobol and RPG)

http://www.bantotal.com/?lang=en

https://en.wikipedia.org/wiki/GeneXus