The language on Firefox is determined by the language you download the installer for. So, if you download it from the mozilla en-us url, you will get the english firefox. To download the right Firefox in your language go to https://www.mozilla.org/en-US/firefox/all/#product-desktop-release to pick the language and download the right Firefox.
If you have downloaded Firefox, you can go to about:config in your firefox url bar and set these to true:
intl.multilingual.enabled
intl.multilingual.downloadEnabled
Warning: I'm not sure if this can make the UI crash, proceed at your own risk!