This is quite interesting.
First of all, for a new account, if the first login is with IMAP, the folders should actually always be created with their English names on the server (as visible through omcontain); the fact that SWA shows German names is just a re-mapping of Special' folders names that happens on the client.
The only way that I know that would lead to non-english folder names to be created would be if the first login to a mailbox is via a non-english version of Outlook; this creates the folders itself and tends to do so in localized form.
From that moment onwards, Outlook and SWA will show names of special folders always appropriate to their UI language.
The per-user language setting (omaddu/ommodu -l or SAC) should not make a difference; this is only for system-generated messages like NDNs or similar sent to the user.
The change in 11.4.3 was to also expose English names only for normal IMAP clients and for web services, such as CalDAV. This was to resolve the situation where a mailbox was indeed created by Outlook with non-english names and then accessed from CalDAV or similar; the URLs in CalDAV, which are based on IMAP folder names, were messed up in this case, so this change was designed. It should not have made a difference for SWA at all.
The only thing I can think of right now is that the "old" foldernames are cached somewhere and because this is an upgrade, something went wrong; the solution for this may involve clearing the IMAP cache of affected users - this is described elsewhere on the forums and in the Wiki. It would, however, not explain the situation with a new user.
Can you positively confirm that, if you
- Create a new user using omaddu -n "Joe User/mailnode"
- login to this user once using SWA
- then look at the user account using omcontain
you see non-english folder names, ever, with no Outlook involved at all?
I would have no immediate explanation for this behavior.
Florian