Firefox add-on to switch a tab's ACCEPT_LANGUAGE

Question

Some non-English websites show a dumbed-down/outdated English version to all visitors whose browsers identifies them as English readers. I usually browse in English but I can read more languages, so I would like to have an easy way to say "Consider me as a Japanese person" for instance.

Requirements:

• Sets the ACCEPT_LANGUAGE HTTP header to the language of my choice, in all requests sent within that tab.
• Does not change the Firefox menus/etc. Only the content of the web page should use the language, not Firefox itself.
• No need to download "language packs".
• Free, ideally open source.

I tried:

• Quick Accept-Language Switcher: Good but applies to the whole browser, not just for one tab.
• Simple Locale Switcher: It does the opposite of what I want: It only put the Firefox menus/etc in Japanese, but keep sending ACCEPT_LANGUAGE en-US,en;q=0.5.
• TabLang: Crashes at every start.

Answer 1

I'm using Quick Locale Switcher for that, and am quite satisfied with it. This addon fits all your requirements, also the implicit one: As below screenshots show, you can configure for which URLs a specific language should be used.

^{Screenshots: Menu, config, config for auto-switch per URL (click images for larger variants)}

Works fine for me here, though it hasn't been updated in a while. There's a somehow more recent "spin-off" by the name Quick Locale Switcher 2, but that still carries a very low version number (0.0.1) and obviously is not yet complete (e.g. auto-switching currently does not work). Still worth keeping an eye on in case it catches up – one never knows what time Mozilla decides to drop some compatibility…

Update: While the former is no longer available at AMO, the latter still is. I've updated its URL with this edit. It's now at v0.1, but wasn't updated since 10/2018.