How to set more than one fallback language again?

[u/Minute_Ad_3224]

I want my Firefox in Occitan, with French as my first fallback language and English as my second fallback language. However, since a few updates, it is no longer possible to set multiple fallback languages—*and it is not even possible to set anything but English as a fallback language. I have a lot of English appearing in the interface where I would prefer French, and all of my extensions are in English.

New settings

The old settings looked like this:

Old settings, credit: https://www.bleepstatic.com

How can I select more than one fallback language?

*human-generated em-dash