Use language or userlanguage (IE) depending on what's defined in the browser.

sdk/lib/html/scripts/systemhtml.py

 #!/usr/bin/python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 import os
@@ skipping 11 matching lines @@
     'Element.insertAdjacentText',
     'Element.remove',
     'ElementEvents.mouseWheel',
     'HTMLCanvasElement.getContext',
     'HTMLSelectElement.options',
     'HTMLSelectElement.selectedOptions',
     'HTMLTableElement.createTBody',
     'IDBDatabase.transaction',
     'MouseEvent.offsetX',
     'MouseEvent.offsetY',
+    'Navigator.language',
     'URL.createObjectURL',
     'URL.revokeObjectURL',
     'WheelEvent.wheelDeltaX',
     'WheelEvent.wheelDeltaY',
     'Window.cancelAnimationFrame',
     'Window.document',
     'Window.indexedDB',
     'Window.location',
     'Window.open',
     'Window.requestAnimationFrame',
@@ skipping 939 matching lines @@
     for library_name in libraries:
       self._libraries[library_name] = DartLibrary(
           library_name, template_loader, library_type, output_dir)
 
   def AddFile(self, basename, library_name, path):
     self._libraries[library_name].AddFile(path)
 
   def Emit(self, emitter, auxiliary_dir):
     for lib in self._libraries.values():
       lib.Emit(emitter, auxiliary_dir)