WordPress is written in PHP, but Automattic (the company that develops WordPress) has other products that are written in other languages.
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.