Firefox ignores system language preference on Mac OS X. Under System Preferences > International, a user can adjust their default language. This allows two users of a single machine to use different languages. For example, with a Japanese version of Mac OS X 10.4 installed, changing the user language to English results in all applications displaying menus and dialogs in English rather than Japanese. Looks like this may be an install related, we only appear to download string resources corresponding to the locale of the install. Safari on the other hand ships with a complete set of string resources, even when the install locale is Japanese.
Mozilla has chosen to deliver its applications in a single language. That's why every language has its own download : <http://www.mozilla.com/en-US/firefox/all.html>. It's also possible to install different "language packs" from <http://releases.mozilla.org/pub/mozilla.org/firefox/releases/188.8.131.52/win32/xpi/>, so that you install multiple languages inside the same executable. There doesn't appear to be an official download page for them though, they're normally only used by the localizers themselves (= the people who produce the translations). You probably need an extension like Quick Locale Switcher <https://addons.mozilla.org/en-US/firefox/addon/1333> to use them. At this moment, the language is not linked to the system language. I don't know if there are any plans to install a real "multi-language" version of Firefox. It's possible though, but the question remains that the extra download size worth it for most users. And what languages should we offer ? The list from Apple Computer is much smaller than the list of Mozilla ! (I thought it as mentioned somewhere in Bugzilla, but I can't find it.
Related issues but not quite duplicates: Bug 356184 Multilingual / multi-language Firefox package for MacOS X Bug 331779 Please enable intl.locale.matchOS by default
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