How to add custom Accept-Languages to Chrome for pseudolocalization testing?

i am testing pseudo-localization of a web-site.

i can configure Internet Explorer to have custom accept languages:

Click Tools, Internet Options
On the General tab click Languages
In the Language Preferences dialog click Add.
Enter a user-defined language of qps-ploc (i.e. the Pseudo (Base) locale)
Click OK

Now when Internet Explorer issues an http request, the accept languages will lead with qpc-ploc:

GET https://stackoverflow.com/ HTTP/1.1
Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, */*
Accept-Language: qps-ploc,en-US;q=0.5
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; InfoPath.3; .NET4.0C; .NET4.0E)
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Host: stackoverflow.com

How do i perform the same change to Chrome? To Firefox?

Update

It should also be noted that Internet Explorer honors my Windows preferences. My Windows is configured to use Pseudo (Base) qps-ploc locale. By default Internet Explorer uses it.

Google Chrome ignores my Windows preferences, deciding instead to request en-US and en language:

GET https://stackoverflow.com/ HTTP/1.1
Host: stackoverflow.com
Connection: keep-alive
Cache-Control: max-age=0
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.782.220 Safari/535.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

In Firefox

Use about:config

(There is a typo in qps-ploc in the screenshots, but you surely get the idea)

In Chrome

(Edit: refer to the comments for ways to avoid Chrome override the setting in new versions.)

Edit the file C:\Users\USERNAME\AppData\Local\Google\Chrome\User Data\Default\Preferences, and add:

{ ...

   "intl": {
      "accept_languages": "qps-ploc,en-us,en"
   },

   ...
}

For example:

Allard Stijnman

You can also set this via the settings page.

Go to settings (cmd + , on mac, probably cntrl + , on windows) and search for language. Click the "manage languages" link and add your preferred language. After that just drag your language to the top of the list and you're done!