This will be a very crappy answer, but here:

All languages boil down to machine code. For example python is implemented in C, and C then gets converted to machine code. So it goes: Python > C > Machine Code. Or for example Java > Java Bytecode > Machine Code. Every single language boils down to one common language, which is machine code.

Now, let's move on to how they actually communicate. All modern CPUs can run in protected mode, as opposed to real mode. In real mode, your program can access any physical RAM that it wants. That means that chrome.exe can access the memory of explorer.exe.

However, in protected mode, applications cannot do that. In protected mode, chrome.exe cannot access the memory of explorer.exe, they both have their own 'protected' memory.

So, how do programs communicate with each other in protected mode? Well, what actually happens is that the program asks the OS kernel(which runs in real mode) to retrieve some information for it.

For example Windows implements 'pipes' to enable processes to communicate with each other.

A pipe is a section of shared memory that processes use for communication. The process that creates a pipe is the pipe server. A process that connects to a pipe is a pipe client. One process writes information to the pipe, then the other process reads the information from the pipe.

As for Reddit, I don't know much on this topic myself but I'll try to give a general explanation:

Reddit's servers are written in Python.

Browsers use(I don't know how) HTML, JS, and CSS to make and represent a document(web-page) to you.

When you load up reddit.com, you connect to Reddit's Python servers which send you some HTML, JS and CSS code, which make up the page. The browser puts that all together, and voila you've got a page.