In that universe, developers think the universe with multiple languages and frameworks is the cool one.
There are many languages that work on both client and server. Most obviously JavaScript/Typescript, but also Scala and Clojure. If webassembly really takes off you can add Rust to this list.
I'm not sure how a combined framework for server and client side would work.
Ocaml with Ocsigen or Bucklescript/ReasonML
Rails and Opal (does anybody use opal?) or clojurescript and clojure might be close.
Particularly if it's the universe where literate programming took off.
CGI + Perl?
CGI::Application + Perl paid my bills for many years back in its day.