"language" attribute causes exception in IE11

This is a bug connected with https://github.com/Polymer/polymer/issues/466 (perhaps better to wait for Polymer's team response in that ticket)

Would you consider renaming this attribute? Other than that it seems to work fine in IE11.

I tried different names: `language`, `lang`, `syntax`... All of them fail for reason described in https://github.com/Polymer/polymer/issues/466

I was lucky renaming it to `highlight`, but that's quite far from the purpose of this attribute.