I finally found a working solution. First I was using a script (RegUpdateAllUsers/RegUpdateAllHkcuHkcr at master · micksmix/RegUpdateAllUsers · GitHub) which iterates trough all users available on a machine. In my case I have edited the script, so it creates the key mentioned in my previous post for every users hkey_current_user entry. As a result of this, the language popup appears for every >existing< user.
Sadly I have encountered problems concerning my logon script. It works perfectly on Windows 7 Machines, but not on a terminal server. I have bypassed the problem by editing the NTUSER.DAT (regedit --> load hive) of the default user, and manually added the key. Thus the key is created as soon as a new user logs on for the first time.
Since the key is automatically deleted after the popup was answered, the user want be asked again.