In order to force the WebSphere Portal to display a certain langugage the easiest way is to replace the "Accept-Language" header to whatever locale you want but if you have muliple virtual hosts you also have to check for the hostname in the request.
The below lines can be placed in the Apache httpd.conf file in order to add headers based on the conditional rule of the hostname.
SetEnvIf Host \.<server host> force-en
#Header echo Accept-Language <- will return the language.
RequestHeader unset Accept-Language env=force-en
RequestHeader append Accept-Language "en-us" env=force-en
