Outlook - after switching Dutch>English language a part is still Dutch

It is nothing serious, but I've always wondered why a part of Outlook, after switching to English language (language-pack) remains Dutch. The folder tree on the left remains Dutch, toolbar etc. are English, Options are English.

See screenshot
1) All Dutch version
2) After switching to English using English language pack
3) How it should be
4) All other items are English, except the folder tree.

Outlook 365 / Windows 10 x64 English version