I recently installed Linux Mint 19 Cinnamon (x64) and do not know much about it. As you know, Firefox comes preinstalled with this distribution and it is in English regardless the language of the OS.
I went ahead and downloaded Firefox version of the desired language (the file extension is tar.bz2). Since I do not know how to install it, I extracted the files and ran the "firefox" executable file in the folder. So far everything is okay, the Firefox is in the desired language. However, whenever I execute the program from anywhere but the downloaded file, English Firefox pops up.
I do not know if it is related but even I set Google as the default search engine in the downloaded Firefox, the setting goes back to Yahoo or Yandex spontaneously.
Awaiting for your help,
Edited by MinterTheMaladroit, 03 August 2018 - 01:21 AM.