Having consulted at several banks around the world I can tell you that it varies everywhere, but the main technologies are usually JSP, .NET and PHP. There isn't really a preferred or set standard technology for their websites, and it will depend on the greater technology focus inside the bank. Some banks are more Microsoft focussed, some are more IBM focussed, some more Java, etc.
Most modern languages are all very secure and robust and well performing - however it comes down to how skilful that language is used by the developers and how well the entire system is architected. You can have a system written in any language but if the developers and designers did not know what they were doing it won't be very secure.