hello, you can download firefox in any language you need through: https://www.mozilla.org/en-US/firefox/all/ if firefox changed its user interface language "on its own" this is highly unusual and suspicious so i'd recommend running a scan with multiple different security tools like the [https://www.malwarebytes.org/antimalware/ free version of malwarebytes], [https://toolslib.net/downloads/viewdownload/1-adwcleaner/ adwcleaner] & [https://www.eset.com/online-scanner/ eset online one-time scanner]. [[Troubleshoot Firefox issues caused by malware]]

The English versions for Windows, Mac, and Linux on www.mozilla.org/firefox/all/ are indeed in English. If you download from www.mozilla.org then the stub setup for Windows is not en-US only as you can get a locale that is related to your location of IP.