Which programming language is used to build the backend of steemit?

Steem is a Graphene 2.0 blockchain implementation. Graphene was originally developed for Bitshares. It is open source and mainly built using the C++ programming language. This is the back-end.

The Steemit condenser is the original front-end for the Steem blockchain, although now there are many applications performing this function. Steemit is built using React, a javascript library. It is also open source.

Hivemind, which will be the new "consensus interpretation" intermediary layer for the Steem blockchain is written in Python. It provides an alternative access point for developers instead of the raw blockchain back-end.

You can find the relevant github development pages here:
https://github.com/steemit