Answer:
It depends entirely on what you want to achieve. If you're a Graphic Designer who hands over HTML and CSS to the development team, then there is little need to learn any programming languages, unless that is something you aspire to do. You are already enhancing your value by knowing HTML and CSS, as there are a lot of designers who simply hand over Photoshop files to the dev team - HTML and CSS is their problem!
JavaScript is essentially a 'front end' or 'client side' language. Your browser executes the JavaScript in your webpages on your computer. It can be used to modify HTML and styles on the fly, and also uses a method called AJAX to send and receive data from the server. It makes web pages more engaging, easier to use, and overall can vastly increase the end user experience. Due to fairly recent developments, JavaScript can now also be used as a back end or server side language.
Python, Ruby & PHP are all languages that run on the server. The server processes the code and sends the web pages to your browser, compare this to JavaScript, where the code is sent to your browser, and that code produces or modifies the HTML within your browser itself.
