So, I tested 7579 again with outlook and a user set to french in the webmail
It still creates duplicate folders and use them instead of the correct ones.
I have already reported this previously.
the only change is that on the filesystem the duplicated folders are created with encoded names, previously it was using the spécial chars (accents).

But the problem remains the same. Any user that has his webmail set to french and connects with outlook will see his folders duplicated. I've stopped the test at this point as it's already a serious enough issue that still exists.
Again, please rollback these IMAP translated folders and keep them for a beta branch. We need a stable release with the other bugfixes requested long time ago.
This is so unprofesionnal :(
The custom build should also be shipped with a changelog against the latest release, so we know what to test/expect from it.
EDIT: A really simple way would be, like proposed already, a toggle that disable the translations for IMAP, to have IMAP act like if the user had it set to english. There is no issue with IMAP when the webmail is set to english.
But we do not want to force the users to have their webmail in english so ...
A simple bool toggle that if enabled tell the IMAP code to use english instead of the webmail language for IMAP.
We could at least use the recent builds that carries other needed bugfixes.