Although I’m French, I usually install the English version of every software I use, the main reason being that it’s almost always the latest version available, which includes the latest features and bug fixes. Plus, looking for help online usually results in answers in English.
The only issue I have is with keyboard shortcuts. I use an AZERTY keyboard. The differences with a QWERTY one aren’t exclusively about keys position. For example, all numbers require using the Shift button to type them. It’s not much if I’m editing a text, but it becomes an issue with keyboard shortcuts.
To zoom in with Firefox, you need to type ‘Command +’. It’s probably easy on a QWERTY but it’s impossible with an AZERTY: the ‘+’ sign requires pressing the Shift button. So if I try typing ‘Command Shift +’ or ‘Command =’ (which is the sign below the ‘+’ one), it doesn’t work (and might even trigger another event).
I haven’t tried the French version of Firefox, and I guess it’s more suited for AZERTY keyboards. But I believe that choosing a software’s language shouldn’t be related to one’s keyboard layout.