Impossible to create a postbox

The Path „~/Library/Mail/Mailboxes/XXX.mbox“
could'nt be created.

This sounds like it could be from a permissions problem. Look at the Mailboxes folder at the location mentioned in the error message. (Note that the '~' character represents your home folder.) Click on this Mailboxes folder, choose File -> Get Info in the Finder, and look in the Ownership & Permissions part of the info window. Make sure you've got Read & Write permissions. Do the same with the Mail folder.

Also, check inside the Mailboxes folder and see if there is already something in there named "XXX.mbox" -- if there is, and you're positive that a mailbox of the same name isn't present in the Mail app, move it to the desktop and try again.

Mail has a bug that causes ~/Library/Mail/Mailboxes to sometimes be a file instead of a folder, and that prevents Mail from creating any mailboxes there.

Verify/repair the startup disk (not just permissions), as described here:

The Repair functions of Disk Utility: what's it all about?

After having fixed all filesystem issues, if any, proceed as follows:

1. Quit Mail if it’s running.

2. In the Finder, go to ~/Library/Mail/.

3. Locate Mailboxes. If it’s a file instead of a folder, move it to the Trash.

BTW, you can make your life easier in these forums by going to System Preferences > International, and moving English to the top of the list. The next time you launch Mail, it'll be all in English. You can then go back to the preferences panel and move your preferred language to the top again so that everything else is in that language.

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder, i.e. ~/Library is the Library folder within the user’s home folder.