Hey,
It mainly depends on what your trying to achieve. Python is taking over the entire world right now, so if you want a high-paid job, and want to create games, websites, etc, Python is your main choice.
HTML and CSS are really really powerful for making websites. I suggest you also check out PHP, which is also really cool.
If you want to do Minecraft Development, I suggest you check out Java. If you want to create mods, clients, etc; check out MCP, or Forge. If you want to make plugins or another sort, check out Spigot. They have an API which is really useful.
Of course, if your really good at HTML and CSS, you should stick with it for the time being. Some developers learn a bit of that language, and then quickly switch to another language, and then forgetting the first language they learnt. This is very time consuming and Web Developers are also fairly important these days.
For other programming languages, such as JavaScript, Swift, C, SQL I think you should check it out in your free time. Just try to learn 1 (Or 2 in this case, making websites, like I need to use SQL and Java to make a decent plugin) language and only switch to another when you know you've mastered it.