Which is the best backend language for future?

Please describe, what you are actually looking for. If it is about learning a backend language, it can be a different one compared to the case of using it for a job or project.
If you do not want to learn a new programming language, but get some experience about backend developing, Node JS can be a good choice. Another option is C# with .NET, which also is used a lot.
I think PHP can still be interesting, because a lot of projects/companies are using it. TypeScript instead of JS can also be a compromise of having new and known aspects.
I hope this helps!

No one can reliably predict that, but we can be pretty sure that C#, Java and PHP will stay strong considering how much they're used on the back-end currently. Choose something where you can find the job easily, preferably something you have some experience with already.

GoLang or Rust are good. Also Node.js have a look at nest.js if you want to stay in the JavaScript/Typescript ecosystem. Good documentation and really nice to work with.

It depens on your usecase your knowledge and how much concurrent requests you need to handle. Maybe look up some articles with compares concurrent request handling in multiple languages. But in most cases node.js/rust/golang/php/Java etc. are all able to handle a lot of concurrent request. So in practice for the most projects it shouldn’t matter.

PHP is anice language. it is a backend language. You can use it as a backend.

Definitely C# - constantly imroving and having MS behind it, makes it sure bet!

I'd still bet on JavaScript / TypeScript, but currently Deno feels more ready for the future than Node.js.

Java!

Nobody said Django. So I will. Python/Django is used by Spotify and Instagram as an instance. You can research about it.