The new editing interface (Gutenberg) has been written in react and there are rumours about more of the backend being rewritten in it too.
edit: I meant more of the admin area, sorry.
The backend being rewritten in React? Maybe I'm missing something but I don't think that's at all correct...
The backend being rewritten in anything but PHP would have implications for hosting, so I highly doubt that given you're talking about a significant fraction of the web. It would also be a backwards incompatible upgrade.
In our data division, we stick to Python and Scala, with a little bit of PHP and Javascript for integrations.