howto change the "Accept-Language" header

Hi all,

I need to change the default Accept-Language header for my web test, 
haven't figure out how to do it exactly. I am using canoo-webtest-plugin for grails.

My webtest.properties looks like this:

wt.config.host = localhost
wt.config.port = 8080
wt.config.protocol = http
.
.
.
wt.config.showhtmlparseroutput = true
wt.webtestMonitor.skip = skip

Can someone please let me know how to set the Accept-Language header 
here properly? Many thanks for your help.

Regards,
Daniel