K9 not saving sent mail

When I send a new e-mail it doesn’t get saved in the “Sent” folder which is very frustrating. Every mail program I have used so far does this easily.
I have Android 8.0 and I normally have the language set to German, both in Android and in K-9, but the problem remains if I set both to English. The folder list is mixed, though, and I think that may be the problem. Very strange. The folders that are available in the main menu are: Posteingang, Postausgang, spamverdacht, Drafts, Sent, Trash.
Half German, half English! But it has been that way ever since installing the program 2 months ago. I just updated to the latest version (6.301), but that didn’t change anything.

Can anyone help? I’ve tried setting the folder for sent mail to “sent” and “Sent”, but neither one works.

Addendum: After further trials, it seems like none of the folders except “Posteingang” work. Saving as a draft does not work either. The mail just disappears. And deleted mails do not end up in the “Trash” folder. They just disappear as well.

I just typed the word “sent” into the search function for you. This is the first result:

So I should try to delete the entire account and then recreate it? That was what worked in the link you sent. It’s worth a try.

Update: OK, it worked. Great and thanks! The “drafts” and “Trash” folder work now too. I’m a happy camper.
The folders are still a funny mish-mash of English and German, though. Weird. It appears that the German translation is incomplete. “sent” should be “Gesendete”, “drafts”–>“Entwürfe”, and “Trash”–>“Papierkorb”.

K-9 Mail displays the names of the folders on the server. If the name of your Sent folder is “sent”, that is what the app will display.