Separate language for frontend and backend

In this post I’m actually want to ask developers make updates in wordpress core.
Here’s the situation, I have website on multi site mode like this:
https://www.example.com – russian language website.
https://www.example.com/en/ – english language website.
In admin (settings->general) I have the ability to choose language for each website, but the problem that it enables selected language for both, frontend and backend of the current website. So, what I need, is to have ability to choose language for backend & frontend separately.
Example:
https://www.example.com – frontend in russian language, backend in russian language.
https://www.example.com/en/ – frontend in english language, backend in russian language.

I really need this option, because it’s not the problem to make separate themes on any language I want, but plugins that have multi language functionality and works with displaying some information on the frontend, gonna show me the language that selected in the admin.

I hope developers will understand all that I’m trying to say, and sorry for my bad knowledge of english, cause it’s not my native language. ??

Thanks!