Drawer sidebar Sent and Trash not translated to another language any guides on issues!!
Which version of K9 do you use? For me on Beta 6.713 I see different icons for Sent and Trash folder. But still I also have english names albeit I use German UI - I guess there just the folder name as it is provided by mail provider is displayed. But yes, I agree, for known folder types the correct translated label could be shown.
Did you set it in Settings / account / Folders? Are there the correct folders selected for transh and sent mails?