Grails internationalization testing

I had some problem with internationalization with Grails.
I tried to change the language by adding ?lang=us at the end of my request but without success.
I found a simple solution : just switch the language settings of your browser by adding first the language you want. It will change "Accept language" in the HTTP request.

For example, in Firefox, you have to go in Tools -> Option -> Content -> language then choose the preferred language.
After that, you only have to reload your web page.