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Firefox has a list of built-in languages that are used to set the HTTP_ACCEPT_LANGUAGE header. How do I add a custom one that's not on the list?

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Is this what you're looking for?

Tools -> Options -> Content ->  Languages -> Select a language to add

It's from a similar post.


As far as I can tell, this GUI option just changes the preference "intl.accept_languages" in about:config. You could edit this yourself to change it to a custom value. I'm not sure if you can put language files somewhere to get a custom language to work or not.

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How do you add an unlisted language? – MItch Jul 8 '10 at 21:06
@MItch I just typed in the two letter code for the language and it worked for me. – AndrejaKo Feb 27 '11 at 10:35

You can use about:config to edit the list of language codes:

Enter about:config into address field, find the setting intl.accept_languages. Its value is the list of language codes to send as Accept-Language HTTP header. The list is comma-separated in descending priority. You can enter any code you like; the codes will even be displayed in the standard pref dialog (see mouche's answer), where you can delete them later (just not add them).

As an aside: Why do you need this? I have never encountered a website that supports more than the (extensive) set of standard tags as defined in ISO-639. Which special tags do you want to use, and why?

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Firefox has a section on their website where you can download and add custom languages to your Firefox:

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How do you add an unlisted language? – MItch Jul 8 '10 at 21:06
-1 No, that's not for adding languages to the list of accepted languages, that's for changing the language of Firefox' UI. – sleske Jul 8 '10 at 21:12

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