I don’t have much time right now but have to ask, why do the language packs not work, I’ve logged in, downloaded the pack, re logged in , tried to enable but there is no option for it, the language just stays on American English. I’m British English (we spell everything correctly!) so I really do not want to use the American one. I’m sure it used to work, but it just randomly changed without me noticing, as things generally do with Firefox
Can you try following the steps on https://support.mozilla.org/en-US/kb/use-firefox-another-language?
Basically the prefs now do most of that for you, aka, find the language pack, download it, give you a restart button.
Worthwhile noting that this only works on beta and release channels, and I think on dev edition. We still need to do more engineering work to be able to do this no Nighlty.