Python. XML, HTML...
Source code and more than you ever want to know can be found here:https://github.com/reddit/reddit
what language is the web design written in?
How can you find languages of any website?
View Page Source
Read through the code.
See references to Python, XML, etc....ergo, it is at least partially written in Python.
Further, click on the link at the bottom which leads to the Source Code at github.
Download, and you can read (and use) the entire source.