2 Answers

1
Go to Mozilla Firefox's FTP website (see Resources below). Click on the folder labeled with your version of Firefox. If you're not sure which version you're running, click on Help and About Firefox from the browser's menu.

2
Choose the folder labeled with your operating system. Select the xpi folder within it.

3
Identify the xpi file of your preferred language. You may need to know the abbreviation of your desired language. If you see multiple xpi results for your preferred language, identify the abbreviation or label of your region.

4
Select the xpi file that corresponds to the language you want to install in the Firefox browser. Verify by clicking Install Now.

5
Restart Firefox to see your changes take effect.

FTP Website >> http://releases.mozilla.org/pub/mozilla.org/firefox/releases/ 

Source: http://www.ehow.com/how_2315722_change-language-packs-firefox.html#ixzz1scgqY75F 

What browser is it?