(I worked in financials/trading systems for many years, in several languages) Since you're already talking on such a scale, I'll assume that you have enough hardware horsepower to drive whatever you need to be competitive, so I'll eliminate the question of execution speed from the discussion.
Readability you already mentioned - and that is indeed important. BUT, not because you're going to write a great work of literature. The reason readibility is important is because it's part of maintainability and agility, which are what you really need to consider more than anything else.
The key is that in such an environment, you can't take a lot of downtime to fix things - you must be able to fix any bug quickly and correctly with minimal downtime and inconvenience to your users. (A trader whose system is down can be very dangerous, especially around developers... LOL [but not too loud] )
And this has more to do with coding style, developer mindset and development infrastructure - version control, carefully versioned and documented releases that you can get back to and patch, etc - than it does with any particular language.
So I think your question is slightly off point. It's not a question of language so much as of developer/management culture and dedication.
But if you want to talk language, I'd say Python/PyQt is the way to go, with the hardware available today.
I make my living writing Delphi code - but if I had a choice, I'd be writing everything in Python. When I code in Delphi (or even more so C++) or even C#, I feel like I'm spending at least half of my time on things that Python would take care of automatically - things that are irrelevant to the program itself - grunt work or even things that require great expertise but that I should not have to deal with.
How much code do you need to write in C++ to manage memory and keep things type safe? How many bugs are introduced because of the difficulty in managing these and similar issues. It's amazing when you think about it - don't use a 20th century tool in the 21st century. That will decrease your maintainability and agility and could be lethal.