Actually, the backtrace goes through the el-get loaddefs file, so maybe something is going wrong there. Let me check with the el-get developers and see if they know what is going on.
Yes, that's very likely. I have been building Org from master branch and it has been working fine.
C-h v org-src-lang-modes tells that it is defined in org-src.el.
So you probably have a mixed installation where an old org-src.el is loaded which doesn't have that variable defined.