User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:69.0) Gecko/20100101 Firefox/69.0
Steps to reproduce:
With thunderbird 68.0-1 (Archlinux), names of folders that are predefined (not user created) appear in English rather than in French (Inbox, Drafts, Sent, Junk, Trash, Outbox).
The thunderbird menu, preferences, home page etc all appear correctly in French (except the menu item "Events and Tasks", but that is related to the lack of localization of the Lightning calendar, a distinct issue).
Downgrading to thunderbird 60.8.0-1 (latest version I have in cache) solves the problem.
I firts reported the bug to the Archlinux bugs, but response was that this was not a packaging issue and I should report it upstream.
See screen capture appended.