You're getting the locale XPIs from Ubuntu. If you go to Tools -> Add-ons, click Locales, and then uninstall next to XULRunner (en-GB), you should be able to remove the en-GB locale.
You're getting the locale XPIs from Ubuntu. If you go to Tools -> Add-ons, click Locales, and then uninstall next to XULRunner (en-GB), you should be able to remove the en-GB locale.