Add the Firefox Profile preference intl.accept_languages. 
Ex.: profile.setPreference( "intl.accept_languages", "no,en-us,en" );

Activate with parameter 'languages' in the testng.xml.
Ex.: <parameter name="parameters"
value="browser=firefox;languages=pt-br,pt-BR;screenshotFolder=/var/tmp/screenshots"
/>
Code and JavaDoc improvements. A better way to get an attribute value.