Hi Uwe,
there is no possibility yet to change subjects and levels without breaking a part of the community hub feature. As you mentionned, there is an issue for that: MDL-23217
You can vote for it.
PS: you could hack the "lang" file (lib/edufields.php) for every sites and hub, but all communication with no modified hub/mooch will be broken. Plus your update process to get security fixes could be a bit less easy if you change core files.