I can't seem to change the language of the browser. There's a language section in the settings but the language won't change when I choose another language.
I've tried to restart the browser and also restart my computer but the language still hasn't changed.
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.enabledintl.multilingual.downloadEnabledWarning: I'm not sure if this can make the UI crash, proceed at your own risk!