Is Django a good choice for a security critical application?
I find your connection between Java and banking wrong ended.
Most Banking Software has terrible security. And much banking software is written in Java. Does ths mean Java makes it more difficult to write secure software than other languages?
Probably it's not Java's fault that there is so little quality security (and safety) wise in Banking software. Actually, like the other posters mention, the choice of your Language usually has very little consequences for your security - unless you select one of the few languages where only hotshot coders can write secure code in (C and PHP come to mind).
Many huge E-Commerce sites are written in Python, Ruby and Perl using various frameworks. And I would argue that the security requirements for merchants are much higher than the requirements of the banking industry. That is because merchants have to provide security and good user experience, while banking customers are willing to put up with unusable interfaces SecureID tokens and whatever.
So yes: Django is up to the task.
Is Django a good choice for a security critical application?
I am asking this because most of the online banking software is built using Java. Is there any real reason for this?