Solved (at least until further notice)
It seems to be a language issue indeed. I had to clean up the languages in several settings
- I had already set the Gmail display language to English (US), but that did not do the trick
- The primary language of the google account had been set to English (Switzerland). I set it to English (US) and the 'Bin' Folder in Gmail was renamed to 'Trash'
- The folder or rather label that had been [Gmail]/Trash became [Gmail]/Bin and I was able to remove that label without issues
- Removing the IMAP folder path of [Gmail] and re-subscribing the IMAP folders in Outlook then brought things back to normal. Trash now seems to be trash, with the empty folder option in the right-click context menu
Diane, thanks for all your help!