RPX language set

I have to put this "language_preference=lang" in a GET value.. To get the RPX plugin translated.
I’ve googled it but have no idea what that is and how to do this.
This is their reaction,
http://groups.google.com/group/rpx-developers/browse_thread/thread/56eb617c8494e3c5/97e7512c14be03fb#97e7512c14be03fb
and besides it’s rude.. it’s non-informative.
So I hope anybody of you would like to explain to me how I can get this done.

Thanks!