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

sdk/lib/html/templates/html/dart2js/impl_Navigator.darttemplate

Index: sdk/lib/html/templates/html/dart2js/impl_Navigator.darttemplate
diff --git a/sdk/lib/html/templates/dart2js_impl.darttemplate b/sdk/lib/html/templates/html/dart2js/impl_Navigator.darttemplate
similarity index 67%
copy from sdk/lib/html/templates/dart2js_impl.darttemplate
copy to sdk/lib/html/templates/html/dart2js/impl_Navigator.darttemplate
index 5c62c0e1b17fad38a846d13a910f39acc673abdb..9b7117f0c5f5b69bac111c63166432087442d0b3 100644
--- a/sdk/lib/html/templates/dart2js_impl.darttemplate
+++ b/sdk/lib/html/templates/html/dart2js/impl_Navigator.darttemplate
@@ -6,4 +6,9 @@ part of $LIBRARYNAME;
 
 /// @domName $DOMNAME; @docsEditable true
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
-$!MEMBERS}
+
+  /// @domName Navigator.language; @docsEditable true
+  String get language => JS('String', '#.language || #.userLanguage', this,
+      this);
+  $!MEMBERS
+}