Firefox - Change language
If you need for some reason to change the Firefox language without re-installing the browser
there is very simple three steps
In the address bar of Firefox, type: "about: config" and then press "enter"
Paste (ctrl+v) in the search bar the following string: general.useragent.locale
Double-click it and then enter, for example "en" or "en-US" to set it to English. type "cs" for Czech, for French "fr" or "fr-FR"
Language pack - Firefox for all operating systems Index of /pub/mozilla.org/firefox/releases/latest/win32/xpi
Locale-Switcher Extension for Firefox