For most applications you will need some sort of data-driven back-end. SQL databases are the most widely used data storage option. Learning SQL therefore will be extremely beneficial.
As for back-end / server technologies and languages again this depends on your background and your environment. Java / Spring / Spring Boot is a very solid option, as are Ruby / Rails or Python / Django.
For Python for backend start super simple for small projects. Bottle.py or CherryPy.
Templating is a thing of the past, use Angular or ReactJS for front end development and use async calls for all data loads.
That's what I'd suggest. Python is easy to learn, start with simple things like bottle.py to to make a simple restful service. JavaScript can be a mess if done wrong, but ES6 is a game changer. I like both Angular and React, I find Angular 1.x super easy to use and learn but never use it in produciton.
If you're starting out, that's a quick dev cycle and easy to learn.
These days, my recommendation for starting with HTML + CSS + Javascript would be https://freecodecamp.org It is not perfect and is not for everyone. But it is a reasonable starting point and exposes people to the inherent complexity of web development in a measured and structured way.
Good luck.
PyRu is/are still good enough, and Go is too, but considering the first sentence, just go with JS.