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

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!

Or direct access to languages settings: chrome://settings/languages

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: