This problem can be very exasperating.I had the same issue English(US) language pack which I couldn't delete in my French Thunderbird version. No guarantee: you may open the Add-ons Manager tab, select Languages and try to remove English(US)

I can't remove English, neither from add-ons nor from the Thunderbird user interface. I have Danish as first language and English as second. My folder pane is now displayed in English. it was in Danish until I put English on as second language. Now I can't get Danish back, because I can't remove English! But thanks for your suggestion.

Now I finally had English removed in Settings under "Language & fonts, Choose the languages used in the Thunderbird user interface", so I only have Danish as language now. But my folder pane is still in English and not in Danish, as it was earlier. How do I get Danish back in my folder pane?

?????

Did you update to v144.0 (release channel- yesterday or today? This version contains a new bug: all special (system) folders except for Inbox have English names (image).

Yes, I did. My folder pane looks like yours. Is there a way I can go back to the earlier version?

In your actual Thunderbird installation first go to Settings > General > Updates and disable automatic updates. Then go to Help > Troubleshooting Information > Profile Folder > Open folder, then quit Thunderbird. In the profile folder look for the file compatibility.ini and delete it.

Uninstall v144.0, download the Danish v143.0 from https://ftp.mozilla.org/pub/thunderbird/releases/143.0/ and install it. Start TB, your folder names should be in Danish again.

I did exactly as you described, and it worked brilliantly! My folder names are now in Danish again. You have been very helpful in solving my problems, and I am very grateful. I would not have found out myself. Thanks so much, Mapenzi!

Thanks so much, Mapenzi!

You're welcome! Would you mind to mark my response from today 8:42 AM as "Helpful" ?