WordNet (r) 3.0 (2006):

source language
    n 1: a language that is to be translated into another language

The Free On-line Dictionary of Computing (30 December 2018):

source code
source
source language

    (Or "source", or rarely "source language")
   The form in which a computer program (software) is written by
   the programmer.  Source code is written in some formal
   programming language which can be compiled automatically into
   object code or machine code or executed by an interpreter.

   Source code might be stored in a source code management system.

   If you have the source code for a program rather than just its
   compiled, executable form, then you can, with the right tools,
   modify it to fix bugs or add new features.  This is the basis
   of the open source philosophy - empowering people to improve the
   software they use for the benefit of themselves and others.

   The Jargon File would have us believe that an old-time hacker
   might refer to source code informally as "English", with the
   implication that to him his favourite programming language is at
   least as readable as English.

   (2014-06-27)