Try editing the following preference in the about:config preference editor.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste intl and pause while the list is filtered

(3) Double-click intl.accept_languages to open a small dialog where you can edit the text sent to the server.

Does it work??

Thanks. But isn't that the universal setting? I would like to have an optional order different from the universal order.

Yes, that is the default setting for all requests.

I don't think there is a per-site setting available as a native feature. Perhaps an add-on offers that, or at least an easy toggle?

You can consider to use the PrefBar extension that comes with a Language menu list to change the intl.accept_languages pref to select a specific language order.

• PrefBar: https://addons.mozilla.org/firefox/addon/prefbar/
• http://prefbar.tuxfamily.org/help/buttons.html#prefbarlanglist

Hi, I found the add-on. It's called Quick Locale Switcher. It's Auto Switcher can remember locale in settings. It can also change locale base on top level domain.