Deprecate NbConvertBase.default_language config option

docs/source/whatsnew/pr/nbconvert-highlight-config-deprecated.rst

@@ -0,0 +1,3 b''
+- Setting the default highlighting language for nbconvert with the config option
+  ``NbConvertBase.default_language`` is deprecated. Nbconvert now respects
+  metadata stored in the :ref:`kernel spec <kernelspecs>`_.

IPython/nbconvert/utils/base.py

@@ -34,7 +34,8 b' class NbConvertBase(LoggingConfigurable):'
                     """
             )
 
-    default_language = Unicode('ipython', config=True, help='default highlight language')
+    default_language = Unicode('ipython', config=True,
+       help='DEPRECATED default highlight language, please use kernelspecs instead')
 
     def __init__(self, **kw):
         super(NbConvertBase, self).__init__(**kw)