Usually you have a client/server architecture where HTML for the site is generated on the server by a multithreaded language. Practically any language can be (and is) used for this, like Perl, Python, PHP, C++, Java, Go, Ruby, C#, Scala, Clojure, Erlang, etc. It's to the point where it feels like literally every language has been used for web dev somewhere. And of course, Javascript, too.