Open the Mozilla Firefox browser.
Click the Menu icon in the upper-left corner of the browser window.
Select Options from the drop-down menu that appears.
Under the Languages and Appearance section click the Choose button.
In the window, click the arrow on the right side of the box (A) that says Select a language to add and then click the Add button (B).
Click the newly selected language (A) and then click the OK button (B).
You'll see the new language when you restart your browser.