What to KnowMenu > Options > General > Language and Appearance > Language > Set Alternatives > Select a language to add > and choose.Shortcut to preferences menu: Type about:preferences in the search bar.Press OK to confirm your language choice.
This article describes how to change the default language in Mozilla Firefox to any of the more than 240 the browser supports.
How to Specify Preferred Languages in Firefox
Setting and modifying Firefox's list of preferred languages can be done quickly. See the steps below:
In Firefox, select the menu icon (the three horizontal bars) in the upper-right corner.
You can also type about:preferences into the Firefox address bar.
In General preferences, scroll down to the Language and Appearance section. Under the Language subheading, you should see your default language. Select Set Alternatives.
Select the Select a language to add drop-down menu.
Scroll through the alphabetical language list and select the language of your choice. To move it into the active list, select Add.
When you're satisfied with your changes, select OK to return to the Firefox preferences. Once there, close the tab or enter a URL to continue your browsing session.
Your new language is added to the preferences list. By default, the new language goes at the top of the list. To change its order, use the Move Up and Move Down buttons accordingly. To remove a specific language from the preferred list, select it, and select Remove.
Firefox displays the first language as your default while browsing. Alternate languages are displayed, if necessary, in the order they appear on this list.
Not all web pages are available in all languages.