In Thunderbird there’s a difference between the IMAP folder name on the server and the name that Thunderbird uses to display that folder in the UI. The folder name on the server is usually ascii-only. The folder name in the UI changes depending on locale, because Thunderbird allows you to change folder semantics (Inbox, Sent, Drafts, Trash, etc) and thus it is able to pick a localized display name for a folder.
Is it possible to make K-9 do the same?
Right now K-9 only localizes the “Inbox” folder (either the unified inbox or the separate inboxes of each account).