If you go to https://www.mozilla.org/sl/firefox/all/#product-desktop-release and you want to download Firefox the website will offer you a download in your language but if you by accident install Firefox in another language, you can always change the language to a new language.
Open Options in your Firefox browser:
The new windows will be opened scroll down to Language and Appearance and change Firefox to a new language.
If your language is not listed inside dropbox simple click Search for more languages.
The new windows will be displayed open dropbox select new language and click Add button.
After that, you will see the progress bar label Downloading….. just wait until the download is finished and a new language in listed inside the table.
After clicking OK you must click the button Apply and Restart. Firefox will be closed and started again in a new language.