Firefox is in German, how can I change the language into Japanese?

Question

I remember I installed it in Japanese, but recently I have downloaded some program which is to be updated following warnings informed by UpdatStar Gmbh, suddenly the language has changed into German. And all of guidance to daownload other programs are in German. So that I can do nothing....because I cannot understand any of German.
I am Japanese, so that Japanese is most convieniet for me, but I can undestand English, so change German into English is acceptable.

Answer 1

If you have the Firefox user interface (menu bar) in the wrong language or want to change the current language then you can get Firefox in the language of your choice here:
* http://www.mozilla.com/firefox/all.html
Uninstall the current Firefox version, but make sure that you do not remove your personal data.<br />
Install the new Firefox version of the wanted language.
You may need to (re)set the pref "general.useragent.locale" to the preferred language (e.g. "en-US" or "en-GB" or "ja") on the about:config page.<br />
You can open the about:config page via the location bar, like you open a web site.<br />
Use the Filter bar at the top of that page to quickly locate the pref.