What is the best programming language for trading expert programming?

The problem is that there isn't any one "final answer" as to which language to use.
If there was, we would all be using it, and not have the plethora of languages we currently have.

I'd suggest that you start by looking at what you know already: languages, and the market you are trying to produce an app for, and then think about how you solve whatever your (very vague) problem is in overview terms rather than starting with a language and trying to build up from there.

And to be honest, the framing of your questions here so far would lead me to think that you are a relative beginner in development, and the task you are trying to achieve is not a beginner project. Heck, it's a pretty specialised subject and you need to know a good amount about how trading works (and it's not the same in all trading markets) and probably a whole host of other things as well before you can automate it: security is an obvious one, and Ai is probably another. And this is probably not a "one man job" - I'd want a team involved, with good cross checking and management to prevent errors "going live".

Remember, this will be dealing with real money, so criminal attention is quite likely, as are legal restraints, as well as the results of small errors being financially catastrophic.

I'd strongly recommend that you need to get quite a lot of ground work under your belt before you even start thinking about starting a project like this. I'd probably need a year or so before I started detailed planning for such an app myself!