Of course you can use it. But some website use another way for languages setting. I should say that I use this method only for setting user agent and all works fine.
In the documentation exist example how to use it.
JavaScript:
instance.SetHeader("HTTP_USER_AGENT", "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0; ZennoPoster)");
I think that you incorrectly set the first parameter. Maybe it should be like this: