If you use multiple languages at the same time in Firefox, there is a way to switch languages automatically. For example, I use Firefox on English and Greek websites and I want to change the Firefox UI from English to Greek and vice versa, depending on the language I write.
Below we describe the fastest way to do this.
All you have to use is a light addon called “Simple Locale Switcher. "
Press together the three Ctrl + Shift + A keys to open the Add-ons panel on a new Firefox tab. (more Firefox hotkeys from here)
You can also click on "Add-ons" from the Tools menu to open it.
In the search box, type Simple Locale Switcher and press Enter.
Click Install, and restart Firefox
Now, click the menu button that looks like a sandwich in the upper right corner of Firefox and select Customize.
Drag the language box to the Firefox menu for faster access
Now you can click on the box from the menu to quickly switch to Firefox languages.
Install the additional languages you need and you're ready!