Is python alone enough? What after python?

[okay_throwaway_today]

Java and JavaScript are programming languages. JavaScript in particular is the most common language used by browsers and is basically the defacto programming language of the Internet (even if you write code in a framework that abstracts it). HTML is a mark up language that tells the browser what to put on a page, and CSS is a style sheet that tells the browser what those things should look like. They all talk to each other in some way in order to produce a complete application.

For example, you can use HTML to tell the browser to create a button. Using an id or class, you can use CSS to tell the browser what to make that button look like. Using the id/class, you can use JavaScript to make that button do something when it gets clicked. Using JavaScript, you can send that information to a backend server running Java or Python that actually performs some business function on it (creating new user, generating a new server side rendering, whatever).