This fits for labels and icons and doesn't work with shared folders too

Well, what is special for one user don't have to be for the other. I could agree with an icon change, but not label.

may be, but the view is the view for this certain user, so if user A uses special folders of user B nothing would hurt user B because the special folder mapping of user A would be required here.

So if user A is of the opinion to map "Junk" to "Cake" and "Trash" to "Berth" he would not only be happy to see other users "Junk" as other users "Cake" and their "Trash" as their "Berth". It would not be required to get other users mapping for those special folders. Because other user "B" mapped "Trash" to "Snug berth" that would not help user A.

So special folder name mapping would always be:

I missed that we actually can't get specific folder type of other user folder because it is stored in a private folder annotation. I don't really like the idea of recognizing folders by their name, which as I understand, you propose.

Do I understand correctly that you proposition is: "If my drafts folder is "Drafts" in IMAP, localize all "Drafts" folders of other users as "Entwürfe" (depending on my lang)"?

No I didn't think about recognizing it by name but thought about getting the annotations.
If you say that's impossible to find out if a folder of another user is by anotation his drafts folder and should be mapped to "Entwürfe" in my german RoundCube Interface because that's what's configured in Settings->Settings->Special folders.
This is relevant especially for languages which are not so popular as english is.
I would have a real problem to find out which of the french terms means "Entwürfe" but it would be possible to map that via the annotation becaus those are lang independent.

Folder subtype annotation is stored in private namespace, you do not have access to it. Also, Archive has no folder annotation in Kolab. So, we can't use folder annotations.